[Xamarin.Android.Build.Tasks] use aapt2 option to generate proguard r…

…ules

Context: https://developer.android.com/studio/build/shrink-code#configuration-files
Fixes: http://work.azdo.io/825420
Fixes: dotnet#3636

Let's say you used the following code fragment in an Android layout:

    <android.support.v7.widget.Toolbar />

Currently, you would need to add a ProGuard rule regardless if you
were using ProGuard or R8.

Fortunately, `aapt2` has a command-line switch:

    > aapt2 link -h
    ...
    --proguard arg    Output file for generated Proguard rules.

According to the Google docs for R8:

    AAPT2 generates keep rules based on references to classes in your
    app’s manifest, layouts, and other app resources. For example, AAPT2
    includes a keep rule for each Activity that you register in your app’s
    manifest as an entry point.

I added a new `ProguardRuleOutput` property for `<Aapt2Link/>` and set
it in cases where `$(AndroidLinkTool)` is not blank. I added this file
to `@(ProguardConfiguration)`, so it will get picked up by ProGuard or
R8.

I updated an existing test for this scenario, checking that a type is
generated in the rules and exists in the final `classes.dex` file.

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Aapt2.targets

@@ -156,6 +156,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
     Condition="'$(_AndroidUseAapt2)' == 'True'">
   <PropertyGroup>
     <AndroidAapt2LinkExtraArgs Condition=" '$(_AndroidUseAapt2)' == 'True' And $(AndroidResgenExtraArgs.Contains('--no-version-vectors')) And !($(AndroidAapt2LinkExtraArgs.Contains('--no-version-vectors'))) ">--no-version-vectors $(AndroidAapt2LinkExtraArgs) </AndroidAapt2LinkExtraArgs>
+    <_Aapt2ProguardRules Condition=" '$(AndroidLinkTool)' != '' ">$(IntermediateOutputPath)aapt_rules.txt</_Aapt2ProguardRules>
   </PropertyGroup>
   <Aapt2Link
       Condition=" '$(_AndroidResourceDesignerFile)' != '' And '$(_AndroidUseAapt2)' == 'True' "
@@ -181,7 +182,12 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
       ToolPath="$(Aapt2ToolPath)"
       ToolExe="$(Aapt2ToolExe)"
       UncompressedFileExtensions="$(AndroidStoreUncompressedFileExtensions)"
+      ProguardRuleOutput="$(_Aapt2ProguardRules)"
   />
+  <ItemGroup Condition=" '$(_Aapt2ProguardRules)' != '' And Exists('$(_Aapt2ProguardRules)') ">
+    <ProguardConfiguration Include="$(_Aapt2ProguardRules)" />
+    <FileWrites Include="$(_Aapt2ProguardRules)" />
+  </ItemGroup>
   <ItemGroup>
     <FileWrites Include="$(IntermediateOutputPath)R.txt" Condition=" '$(_AndroidUseAapt2)' == 'True' And Exists ('$(IntermediateOutputPath)R.txt') " />
   </ItemGroup>

src/Xamarin.Android.Build.Tasks/Tasks/Aapt2Link.cs

@@ -74,6 +74,8 @@ public class Aapt2Link : Aapt2 {
 
 		public bool ProtobufFormat { get; set; }
 
+		public string ProguardRuleOutput { get; set; }
+
 		AssemblyIdentityMap assemblyMap = new AssemblyIdentityMap ();
 		List<string> tempFiles = new List<string> ();
 
@@ -185,6 +187,9 @@ string GenerateCommandLineCommands (string ManifestFile, string currentAbi, stri
 				if (!string.IsNullOrWhiteSpace (assetDir) && Directory.Exists (assetDir))
 					cmd.AppendSwitchIfNotNull ("-A ", assetDir);
 			}
+			if (!string.IsNullOrEmpty (ProguardRuleOutput)) {
+				cmd.AppendSwitchIfNotNull ("--proguard ", ProguardRuleOutput);
+			}
 			cmd.AppendSwitchIfNotNull ("-o ", currentResourceOutputFile);
 			return cmd.ToString ();
 		}

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -1011,7 +1011,7 @@ public void BuildAotApplicationAndBundleAndÜmläüts (string supportedAbis, boo
 		[Test]
 		public void BuildProguardEnabledProject ([Values (true, false)] bool isRelease, [Values ("dx", "d8")] string dexTool, [Values ("", "proguard", "r8")] string linkTool)
 		{
-			var proj = new XamarinAndroidApplicationProject {
+			var proj = new XamarinFormsAndroidApplicationProject {
 				IsRelease = isRelease,
 				DexTool = dexTool,
 				LinkTool = linkTool,
@@ -1023,13 +1023,19 @@ public void BuildProguardEnabledProject ([Values (true, false)] bool isRelease,
 					var proguardProjectPrimary = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "proguard", "proguard_project_primary.cfg");
 					FileAssert.Exists (proguardProjectPrimary);
 					Assert.IsTrue (StringAssertEx.ContainsText (File.ReadAllLines (proguardProjectPrimary), $"-keep class {proj.JavaPackageName}.MainActivity"), $"`{proj.JavaPackageName}.MainActivity` should exist in `proguard_project_primary.cfg`!");
+
+					var toolbar_class = "android.support.v7.widget.Toolbar";
+					var aapt_rules = b.Output.GetIntermediaryPath ("aapt_rules.txt");
+					FileAssert.Exists (aapt_rules);
+					Assert.IsTrue (StringAssertEx.ContainsText (File.ReadAllLines (aapt_rules), $"-keep class {toolbar_class}"), $"`{toolbar_class}` should exist in `{aapt_rules}`!");
 				}
 
 				var dexFile = b.Output.GetIntermediaryPath (Path.Combine ("android", "bin", "classes.dex"));
 				FileAssert.Exists (dexFile);
 				var classes = new [] {
 					"Lmono/MonoRuntimeProvider;",
 					"Landroid/runtime/UncaughtExceptionHandler;",
+					"Landroid/support/v7/widget/Toolbar;"
 				};
 				foreach (var className in classes) {
 					Assert.IsTrue (DexUtils.ContainsClassWithMethod (className, "<init>", "()V", dexFile, AndroidSdkPath), $"`{dexFile}` should include `{className}`!");