[build] Add android-platform-support.override.props (#10025)

Context: dotnet/android-tools@34e98e2

The `external/monodroid.override.props` file has been updated and
renamed to `external/android-platform-support.override.props`, which
causes the `external/android-platform-support` build to use the
`external/xamarin-android-tools` checkout within this repo instead of
its own dotnet/android-tools repo checkout.

This override should ensure that we always use the
dotnet/android-tools path from dotnet/android when building both
android and the debugging tasks, which should resolve the
`Microsoft.Android.Build.BaseTasks` assembly version mismatch issue
we've seen recently when trying to update the dotnet/android-tools
submodule reference without also updating android-platform-support:

	Xamarin.Android.Common.Debugging.targets(74,2): error MSB4062:
	The "Xamarin.Android.Tasks.ResolveXamarinAndroidTools" task could not be loaded from the assembly Xamarin.Android.Build.Debugging.Tasks.dll.
	Could not load file or assembly 'Microsoft.Android.Build.BaseTasks, Version=1.0.96.0, Culture=neutral, PublicKeyToken=84e04ff9cfb79065'.
	The system cannot find the file specified.

Author: pjcollins

external/android-platform-support.override.props

@@ -0,0 +1,8 @@
+<Project>
+
+  <PropertyGroup>
+    <XamarinAndroidToolsDirectory>$(MSBuildThisFileDirectory)xamarin-android-tools</XamarinAndroidToolsDirectory>
+    <XamarinAndroidToolsFullPath>$([System.IO.Path]::GetFullPath ('$(XamarinAndroidToolsDirectory)'))\</XamarinAndroidToolsFullPath>
+  </PropertyGroup>
+
+</Project>