[build] introduce xamarin-android-tools.override.props (#1148)

Context: dotnet/android#8360
Context: 8898bc1
Context: 9a878f2
Context: dotnet/android-tools@34e98e2

When .NET 8 RC 2 took a dependency on dotnet/runtime 7.0.12 and 6.0.23,
we added `external/xamarin-android-tools.override.props` in
dotnet/android#8360 with the contents:

	<Project>
	  <PropertyGroup>
	    <RestoreAdditionalProjectSources>
	      https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-26e0f822/nuget/v3/index.json;
	      https://pkgs.dev.azure.com/dnceng/public/_packaging/darc-pub-dotnet-runtime-301ba1ee/nuget/v3/index.json;
	    </RestoreAdditionalProjectSources>
	  </PropertyGroup>
	</Project>

This allowed xamarin-android/external/xamarin-android-tools to find
and use the new NuGet sources, but
xamarin-android/external/Java.Interop uses its own checkout of
xamarin-android-tools in
`xamarin-android/external/java.interop/external/xamarin-android-tools`.`

This led to the error during the `prepare java.interop Debug` stage:

	tests/api-compatibility/api-compatibility.targets(3,3): warning MSB4011: "Configuration.props" cannot be imported again. It was already imported at "build-tools/scripts/RunTests.targets (7,3)". This is most likely a build authoring error. This subsequent import will be ignored.
	external/Java.Interop/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj : error NU1102: Unable to find package Microsoft.NETCore.App.Ref with version (= 6.0.23)
	external/Java.Interop/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj : error NU1102: - Found 86 version(s) in dotnet-public [ Nearest version: 7.0.0-preview.1.22076.8 ]
	external/Java.Interop/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj : error NU1102: - Found 1 version(s) in dotnet-eng [ Nearest version: 5.0.0-alpha.1.19618.1 ]
	external/Java.Interop/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj : error NU1102: Unable to find package Microsoft.NETCore.App.Ref with version (= 6.0.23)
	external/Java.Interop/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj : error NU1102: - Found 86 version(s) in dotnet-public [ Nearest version: 7.0.0-preview.1.22076.8 ]
	external/Java.Interop/external/xamarin-android-tools/src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj : error NU1102: - Found 1 version(s) in dotnet-eng [ Nearest version: 5.0.0-alpha.1.19618.1 ]
	build-tools/scripts/DotNet.targets(19,5): error MSB3073: The command ""bin/Release/dotnet/dotnet" build -t:Prepare Java.Interop.sln -c Debug -p:JdksRoot= -p:DotnetToolPath=bin/Release/dotnet/dotnet -bl:build-tools/scripts/../../bin/BuildDebug/msbuild-20230925T183954-prepare-java-interop.binlog" exited with code 1.
	    1 Warning(s)
	    7 Error(s)

Introduce an `external/xamarin-android-tools.override.props` within
Java.Interop which imports `..\Directory.Build.props`.  This allows
MSBuild properties to "flow" from a "parent"
`xamarin-android/external/Java.Interop.override.props` through to
`xamarin-android/external/Java.Interop/external/xamarin-android-tools`,
allowing a xamarin-android checkout to more easily control MSBuild
properties used by xamarin-android-tools.

Author: jonathanpeppers

external/xamarin-android-tools.override.props

@@ -0,0 +1,3 @@
+<Project>
+  <Import Project="..\Directory.Build.props" />
+</Project>