[xaprepare] Add AndroidToolchain scenario

Usage: `xaprepare.exe -s=AndroidToolchain`

The step will *only* install Android SDK+NDK as well as the OpenJDK distribution
we provision. No other programs are installed or updated.

build-tools/xaprepare/xaprepare/Scenarios/Scenario_AndroidToolchain.cs

@@ -0,0 +1,24 @@
+using System;
+
+namespace Xamarin.Android.Prepare
+{
+	[Scenario (isDefault: false)]
+	partial class Scenario_AndroidToolchain : ScenarioNoStandardEndSteps
+	{
+		public Scenario_AndroidToolchain () 
+			: base ("AndroidToolchain", "Install Android SDK, NDK and Corretto JDK.", Context.Instance)
+		{}
+
+		protected override void AddSteps (Context context)
+		{
+			Steps.Add (new Step_Android_SDK_NDK ());
+			Steps.Add (new Step_InstallCorrettoOpenJDK ());
+
+			// disable installation of missing programs...
+			context.SetCondition (KnownConditions.AllowProgramInstallation, false);
+
+			// ...but do not signal an error when any are missing
+			context.SetCondition (KnownConditions.IgnoreMissingPrograms, true);
+		}
+	}
+}

build-tools/xaprepare/xaprepare/xaprepare.csproj

@@ -125,6 +125,7 @@
     <Compile Include="ConfigAndData\Runtimes.cs" />
     <Compile Include="OperatingSystems\OS.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
+    <Compile Include="Scenarios\Scenario_AndroidToolchain.cs" />
     <Compile Include="Scenarios\Scenario_PrepareExternalGitDependencies.cs" />
     <Compile Include="Scenarios\Scenario_PrepareImageDependencies.cs" />
     <Compile Include="Scenarios\Scenario_Required.cs" />