Bump to xamarin/xamarin-android-tools/master@a6a23bb5 (#4891)

Changes: dotnet/android-tools@3974fc3...a6a23bb

  * dotnet/android-tools@a6a23bb: [Xamarin.Android.Tools.AndroidSdk] Default SDK component versions (#93)
  * dotnet/android-tools@32a1e2c: [build] fail macOS build if tests fail (#94)
  * dotnet/android-tools@79a0141: Return a default for unknown API levels (#90)

Of particular note is dotnet/android-tools@a6a23bb:

In order to ensure that both Xamarin.Android and the macOS installer
use the same default version numbers for Android SDK components, we are
now using `Xamarin.Android.Tools.Versions.props` to contain default
version number information.  This file is maintained in the
xamarin/xamarin-android-tools repo and will be imported via
`external/xamarin-android-tools`:

	<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
	   <PropertyGroup>
	     <AndroidSdkBuildToolsVersion Condition="'$(AndroidSdkBuildToolsVersion)' == ''">29.0.2</AndroidSdkBuildToolsVersion>
	     …

The new file is imported from `Xamarin.Android.Common.props`.

build-tools/installers/create-installers.targets

@@ -256,6 +256,7 @@
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.Tools.Bytecode.pdb" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.Tools.JavadocImporter.dll" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.Tools.JavadocImporter.pdb" />
+    <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.Tools.Versions.props" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Android.VisualBasic.targets" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Build.AsyncTask.dll" />
     <_MSBuildFiles Include="$(MSBuildSrcDir)\Xamarin.Build.AsyncTask.pdb" />

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.props.in

@@ -1,4 +1,6 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+	<!-- Android Sdk Tool versions are sourced from this file. This is maintained in the xamarin-android-tools repo. -->
+	<Import Project="$(MSBuildThisFileDirectory)\Xamarin.Android.Tools.Versions.props" />
 	<PropertyGroup>
 		<XamarinAndroidVersion Condition=" '$(UsingAndroidNETSdk)' != 'true' ">@PACKAGE_VERSION@-@PACKAGE_VERSION_BUILD@</XamarinAndroidVersion>
 		<_JavaInteropReferences>Java.Interop;System.Runtime</_JavaInteropReferences>
@@ -9,16 +11,11 @@
 		<YieldDuringToolExecution Condition="'$(YieldDuringToolExecution)' == ''">true</YieldDuringToolExecution>
 		<LatestSupportedJavaVersion Condition="'$(LatestSupportedJavaVersion)' == ''">11.0.99</LatestSupportedJavaVersion>
 		<MinimumSupportedJavaVersion Condition="'$(MinimumSupportedJavaVersion)' == ''">1.6.0</MinimumSupportedJavaVersion>
-		<AndroidCommandLineToolsVersion Condition=" '$(AndroidCommandLineToolsVersion)' == '' ">1.0</AndroidCommandLineToolsVersion>
 		<AndroidVersionCodePattern Condition=" '$(AndroidUseLegacyVersionCode)' != 'True' And '$(AndroidVersionCodePattern)' == '' ">{abi}{versionCode:D5}</AndroidVersionCodePattern>
 		<AndroidResourceGeneratorTargetName>UpdateGeneratedFiles</AndroidResourceGeneratorTargetName>
 		<AndroidUseAapt2 Condition=" '$(AndroidUseAapt2)' == '' ">True</AndroidUseAapt2>
 		<AndroidPackageNamingPolicy Condition=" '$(AndroidPackageNamingPolicy)' == '' ">LowercaseCrc64</AndroidPackageNamingPolicy>
 		<AndroidUseManagedDesignTimeResourceGenerator Condition=" '$(AndroidUseManagedDesignTimeResourceGenerator)' == '' And '$(OS)' != 'Windows_NT' ">False</AndroidUseManagedDesignTimeResourceGenerator>
-		<AndroidSdkBuildToolsVersion Condition="'$(AndroidSdkBuildToolsVersion)' == ''">29.0.2</AndroidSdkBuildToolsVersion>
-		<AndroidSdkPlatformToolsVersion Condition="'$(AndroidSdkPlatformToolsVersion)' == ''">29.0.5</AndroidSdkPlatformToolsVersion>
-		<AndroidSdkToolsVersion Condition="'$(AndroidSdkToolsVersion)' == ''">26.1.1</AndroidSdkToolsVersion>
-		<AndroidNdkVersion Condition="'$(AndroidNdkVersion)' == ''">16.1</AndroidNdkVersion>
 		<BundleToolVersion Condition="'$(BundleToolVersion)' == ''">@BUNDLETOOL_VERSION@</BundleToolVersion>
 		<_XamarinAndroidMSBuildDirectory>$(MSBuildThisFileDirectory)</_XamarinAndroidMSBuildDirectory>