Implement network capabilities reporting

This commit should fix the problem where VPN connection is viewed as metered connections on Android P DP1. However as I only have an emulator to test, I can't tell if it really works but it should.

An interesting observation is that the developer can call setUnderlyingNetworks(arrayOfNulls(1)) to make the VPN connection always unmetered. This is more of a quick hack instead of a real fix. Source: https://android.googlesource.com/platform/frameworks/base/+/adbf1d0/services/core/java/com/android/server/connectivity/Vpn.java#312

One may ask whether it's possible to fix this issue for Android 8.1. The short answer is not that I know of.
After some intensive source code reading, I found out that this is the commit that "breaks" whether VPN is metered for Android 8.1: https://android.googlesource.com/platform/frameworks/base/+/43d2a1700b6eb1d804924c6a1e5e0161a13a5348%5E%21/
The key is that isActiveNetworkMetered() used to return getActiveNetworkInfo().isMetered() and now it uses the more up-to-date API getActiveNetwork(). However, getActiveNetworkInfo() ignores VPN connections while getActiveNetwork() prefers VPN connection. And since NetworkCapabilities for VPN Network is always metered by default, this ultimately causes the bug. One might be able to get around this bug by somehow calling NetworkAgent.sendNetworkCapabilities in the correct context.

Fix #1666.

mobile/src/main/AndroidManifest.xml

@@ -3,6 +3,7 @@
           package="com.github.shadowsocks"
           android:installLocation="internalOnly">
 
+    <uses-permission android:name="android.permission.CHANGE_NETWORK_STATE"/>
     <uses-permission android:name="android.permission.INTERNET"/>
     <uses-permission android:name="android.permission.RECEIVE_BOOT_COMPLETED"/>
     <uses-permission android:name="android.permission.NFC" />

mobile/src/main/java/com/github/shadowsocks/bg/VpnService.kt

@@ -20,12 +20,16 @@
 
 package com.github.shadowsocks.bg
 
+import android.annotation.TargetApi
 import android.app.Service
+import android.content.Context
 import android.content.Intent
 import android.content.pm.PackageManager
-import android.net.LocalSocket
+import android.net.*
+import android.os.Build
 import android.os.IBinder
 import android.os.ParcelFileDescriptor
+import android.support.v4.os.BuildCompat
 import android.util.Log
 import com.github.shadowsocks.App.Companion.app
 import com.github.shadowsocks.JniHelper
@@ -50,6 +54,20 @@ class VpnService : BaseVpnService(), LocalDnsService.Interface {
         private const val PRIVATE_VLAN6 = "fdfe:dcba:9876::%s"
 
         private val getInt: Method = FileDescriptor::class.java.getDeclaredMethod("getInt$")
+
+        /**
+         * Unfortunately registerDefaultNetworkCallback is going to return our VPN interface: https://android.googlesource.com/platform/frameworks/base/+/dda156ab0c5d66ad82bdcf76cda07cbc0a9c8a2e
+         *
+         * This makes doing a requestNetwork with REQUEST necessary so that we don't get ALL possible networks that
+         * satisfies default network capabilities but only THE default network. Unfortunately we need to have
+         * android.permission.CHANGE_NETWORK_STATE to be able to call requestNetwork.
+         *
+         * Source: https://android.googlesource.com/platform/frameworks/base/+/2df4c7d/services/core/java/com/android/server/ConnectivityService.java#887
+         */
+        private val defaultNetworkRequest = NetworkRequest.Builder()
+                .addCapability(NetworkCapabilities.NET_CAPABILITY_INTERNET)
+                .addCapability(NetworkCapabilities.NET_CAPABILITY_NOT_RESTRICTED)
+                .build()
     }
 
     private inner class ProtectWorker : LocalSocketListener("ShadowsocksVpnThread") {
@@ -90,6 +108,22 @@ class VpnService : BaseVpnService(), LocalDnsService.Interface {
     private var worker: ProtectWorker? = null
     private var tun2socksProcess: GuardedProcess? = null
 
+    private val connectivity by lazy { getSystemService(Context.CONNECTIVITY_SERVICE) as ConnectivityManager }
+    @TargetApi(Build.VERSION_CODES.P)
+    private val defaultNetworkCallback = object : ConnectivityManager.NetworkCallback() {
+        override fun onAvailable(network: Network) {
+            setUnderlyingNetworks(arrayOf(network))
+        }
+        override fun onCapabilitiesChanged(network: Network, networkCapabilities: NetworkCapabilities?) {
+            // it's a good idea to refresh capabilities
+            setUnderlyingNetworks(arrayOf(network))
+        }
+        override fun onLost(network: Network) {
+            setUnderlyingNetworks(null)
+        }
+    }
+    private var listeningForDefaultNetwork = false
+
     override fun onBind(intent: Intent): IBinder? = when (intent.action) {
         SERVICE_INTERFACE -> super<BaseVpnService>.onBind(intent)
         else -> super<LocalDnsService.Interface>.onBind(intent)
@@ -98,6 +132,10 @@ class VpnService : BaseVpnService(), LocalDnsService.Interface {
     override fun onRevoke() = stopRunner(true)
 
     override fun killProcesses() {
+        if (BuildCompat.isAtLeastP() && listeningForDefaultNetwork) {
+            connectivity.unregisterNetworkCallback(defaultNetworkCallback)
+            listeningForDefaultNetwork = false
+        }
         worker?.stopThread()
         worker = null
         super.killProcesses()
@@ -179,6 +217,13 @@ class VpnService : BaseVpnService(), LocalDnsService.Interface {
         this.conn = conn
         val fd = conn.fd
 
+        // We only need to update since Android P: https://android.googlesource.com/platform/frameworks/base/+/72f9c42b9e59761a28d6b32c42f65de57c98daed
+        if (BuildCompat.isAtLeastP()) {
+            // we want REQUEST here instead of LISTEN
+            connectivity.requestNetwork(defaultNetworkRequest, defaultNetworkCallback)
+            listeningForDefaultNetwork = true
+        }
+
         val cmd = arrayListOf(File(applicationInfo.nativeLibraryDir, Executable.TUN2SOCKS).absolutePath,
                 "--netif-ipaddr", PRIVATE_VLAN.format(Locale.ENGLISH, "2"),
                 "--netif-netmask", "255.255.255.0",