[Xamarin.Android.Build.Tools] Add $(Android*Version) Properties (#1195)

Fixes: #1165

Add the following overridable properties:

  * `$(AndroidSdkPlatformToolsVersion)`: The preferred/default
    version of the Android SDK `platform-tools` package.
  * `$(AndroidSdkToolsVersion)`: The preferred/default version
    of the Android SDK `tools` package.
  * `$(AndroidNdkVersion)`: The preferred/default version of the
    Android NDK.

The Visual Studio (+ for Mac) SDK Manager will eventually use these
values to determine which package versions to install.

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -233,6 +233,9 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 	<GenerateSerializationAssemblies>Off</GenerateSerializationAssemblies>
 
 	<AndroidSdkBuildToolsVersion Condition="'$(AndroidSdkBuildToolsVersion)' == ''">23.0.0</AndroidSdkBuildToolsVersion>
+	<AndroidSdkPlatformToolsVersion Condition="'$(AndroidSdkPlatformToolsVersion)' == ''">25.0.5</AndroidSdkPlatformToolsVersion>
+	<AndroidSdkToolsVersion Condition="'$(AndroidSdkToolsVersion)' == ''">25.2.5</AndroidSdkToolsVersion>
+	<AndroidNdkVersion Condition="'$(AndroidNdkVersion)' == ''">r14b</AndroidNdkVersion>
 
 	<!-- Obsolete -->
 	<AndroidGdbDebugServer>None</AndroidGdbDebugServer>