[Xamarin.Android.Tools.AndroidSdk] Default SDK component versions (#93)

Context: https://issuetracker.google.com/issues/150189789
Context: xamarin/androidtools@3f51412

We have discovered an issue when installing Visual Studio for Mac
on macOS: certain versions of the installer will install the *latest*
Android SDK Build-tools package, currently r30.0.0.

This is a problem because recent versions of Xamarin.Android will use
`apksigner` to sign `Mono.Android.Platform.ApiLevel_*.apk` files, so
that they can be installed on API-30 targets -- API-30 targets no
longer like `jarsigner`-signed `.apk` files -- but the `apksigner`
included in the Build-tools r30 package requires JDK 9.

Visual Studio for Mac is not ready to migrate to JDK >= 9.

Consequently, on such installation environments, the Xamarin.Android
shared runtime cannot be created, as `apksigner` won't run:

	Exception in thread "main" java.lang.UnsupportedClassVersionError: com/android/apksigner/ApkSignerTool
	has been compiled by a more recent version of the Java Runtime (class file version 53.0),
	this version of the Java Runtime only recognizes class file versions up to 52.0

What we need is a way for the macOS installer to share the same
[default Android SDK component versions as Xamarin.Android itself][0].

This *could* plausibly be done by adding a git submodule reference from
the xamarin/xamarin-android repo to the macOS installer, but this is
undesirable.

Instead, both the the xamarin-android repo and the installer repo's
have existing git submodules graphs which include the
xamarin/xamarin-android-tools repo.

Add a new `Xamarin.Android.Tools.Versions.props` file, which contains
the default Android SDK component versions, as an MSBuild project file:

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

With this file in place, we can update the macOS installer to use
`Xamarin.Android.Tools.Versions.props` as the "source of truth" for
which versions to install.  In particular, we'll set the default
Build-tools version to 29.0.2, *not* 30.0.0, thus avoiding the problem
with `apksigner` invocations.

In the future, when `Xamarin.Android.Tools.Versions.props` is changed
*all* repos referencing xamarin/xamarin-android-tools will need to be
updated, so that everything can be on the same "page".

[0]: https://github.com/xamarin/xamarin-android/blob/5f78740d74e19b235aad62c8ea8e314c9c78ddce/src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.props.in#L18-L21

src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.AndroidSdk.csproj

@@ -37,4 +37,9 @@
     </FilesToSign>
   </ItemGroup>
 
+  <ItemGroup>
+    <None Update="Xamarin.Android.Tools.Versions.props">
+      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+    </None>
+  </ItemGroup>
 </Project>

src/Xamarin.Android.Tools.AndroidSdk/Xamarin.Android.Tools.Versions.props

@@ -0,0 +1,20 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <PropertyGroup>
+    <!--
+    This file contains the versions of the tooling which will be installed
+    by default on a users machine. They should be the latest stable versions
+    which xamarin.android works with. 
+    
+    If this file is changed the submodule for androidtools should be updated, 
+    along with any other repo which references androidtools. 
+    -->
+    <AndroidSdkBuildToolsVersion Condition="'$(AndroidSdkBuildToolsVersion)' == ''">29.0.2</AndroidSdkBuildToolsVersion>
+    <AndroidCommandLineToolsVersion Condition=" '$(AndroidCommandLineToolsVersion)' == '' ">1.0</AndroidCommandLineToolsVersion>
+    <AndroidSdkPlatformToolsVersion Condition="'$(AndroidSdkPlatformToolsVersion)' == ''">29.0.5</AndroidSdkPlatformToolsVersion>
+    <AndroidSdkToolsVersion Condition="'$(AndroidSdkToolsVersion)' == ''">26.1.1</AndroidSdkToolsVersion>
+    <AndroidSdkEmulatorVersion Condition="'$(AndroidSdkEmulatorVersion)' == ''"></AndroidSdkEmulatorVersion>
+    <AndroidSdkPlatformVersion Condition="'$(AndroidSdkPlatformVersion)' == ''">android-29</AndroidSdkPlatformVersion>
+    <AndroidNdkVersion Condition="'$(AndroidNdkVersion)' == ''">16.1</AndroidNdkVersion>
+  </PropertyGroup>
+</Project>