android: fix timezone so the BitBox02 shows the correct time

On Android, due to a bug in
Go (golang/go#20455), the local timezone is
not correct. It is always UTC.

As a result, when restoring a backup on the BitBox02, the BitBox02
displays the current time in UTC instead of the local time.

This commit contains a workaround to fix the timezone on Android.

Alternative considered: getting the timezone offset in native Android code and sending it
to Go-land via backend.Environment. I did not pursue this as a quick
search didn't turn up an easy way to get the timezone offset on
Android which takes into account daylight-savings etc.

frontends/android/goserver/goserver.go

@@ -17,7 +17,10 @@ package goserver
 import (
 	"io"
 	"log"
+	"os/exec"
+	"strings"
 	"sync"
+	"time"
 
 	"github.com/digitalbitbox/bitbox-wallet-app/backend/bridgecommon"
 	"github.com/digitalbitbox/bitbox-wallet-app/backend/devices/usb"
@@ -30,6 +33,29 @@ var (
 	once sync.Once
 )
 
+// fixTimezone sets the local timezone on Android. This is a workaround to the bug that on Android,
+// time.Local is hard-coded to UTC. See https://github.com/golang/go/issues/20455.
+//
+// We need the correct timezone to be able to send the `time.Now().Zone()` offset to the BitBox02.
+// Without it, the BitBox02 will always display UTC time instad of local time.
+//
+// This fix is copied from https://github.com/golang/go/issues/20455#issuecomment-342287698.
+func fixTimezone() {
+	out, err := exec.Command("/system/bin/getprop", "persist.sys.timezone").Output()
+	if err != nil {
+		return
+	}
+	z, err := time.LoadLocation(strings.TrimSpace(string(out)))
+	if err != nil {
+		return
+	}
+	time.Local = z
+}
+
+func init() {
+	fixTimezone()
+}
+
 // the Go*-named interfaces are implemented in Java for the mobile client. The "Go" prefix is so
 // that the code is more readable in Java (interfaces coming from Go-land). The implemented
 // interfaces are than translated to implement backend.Environment (see see backendEnvironment