[Xamarin.Android.Build.Tasks] Make manifestmerger.jar the default. (#8392)

Context: #8387

Context: 2c6f5cd
Context: f7ea4a3
Context: d794534

Update the value for `$(AndroidManifestMerger)` (f7ea4a3) to
`manifestmerger.jar`.  This allows users to make use of the new
features like `@(AndroidManifestOverlay)` (d794534) out of the box.

The old system is still available, users will need to add the
following to their csproj:

	<AndroidManifestMerger>legacy</AndroidManifestMerger>

AndroidX has been building with
`$(AndroidManifestMerger)`=manifestmerger.jar for
[over three years][0].

[0]: dotnet/android-libraries@c6c0e50

Author: dellis1972

Documentation/guides/building-apps/build-properties.md

@@ -987,9 +987,12 @@ merging *AndroidManifest.xml* files. This is an enum-style property
 where `legacy` selects the original C# implementation
 and `manifestmerger.jar` selects Google's Java implementation.
 
-The default value is currently `legacy`. This will change to
-`manifestmerger.jar` in a future release to align behavior with
-Android Studio.
+The default value is currently `manifestmerger.jar`. If you want to 
+use the old version add the following to your csproj
+
+```xml
+<AndroidManifestMerger>legacy</AndroidManifestMerger>
+```
 
 Google's merger enables support for `xmlns:tools="http://schemas.android.com/tools"`
 as described in the [Android documentation][manifest-merger].

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

@@ -670,6 +670,8 @@ public void AllResourcesInClassLibrary ([Values (true, false)] bool useAapt2, [V
 			if (Builder.UseDotNet) {
 				lib.RemoveProperty ("OutputType");
 			}
+			lib.AndroidManifest = lib.AndroidManifest.
+				Replace ("application android:label=\"${PROJECT_NAME}\"", "application android:label=\"com.test.foo\" ");
 
 			// Create an "app" that is basically empty and references the library
 			var app = new XamarinAndroidLibraryProject {

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

@@ -755,6 +755,7 @@ public void MergeLibraryManifest ()
 					KnownPackages.SupportV7AppCompat_27_0_2_1,
 				},
 			};
+			proj.SetProperty ("AndroidManifestMerger", "legacy");
 			proj.Sources.Add (new BuildItem.Source ("TestActivity1.cs") {
 				TextContent = () => @"using System;
 using System.Collections.Generic;

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -275,7 +275,7 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 	<AndroidCreatePackagePerAbi Condition=" '$(AndroidCreatePackagePerAbi)' == 'aab' ">False</AndroidCreatePackagePerAbi>
 	<AndroidApkSigningAlgorithm Condition=" '$(AndroidApkSigningAlgorithm)' == '' ">SHA256withRSA</AndroidApkSigningAlgorithm>
 	<AndroidApkDigestAlgorithm  Condition=" '$(AndroidApkDigestAlgorithm)' == '' ">SHA-256</AndroidApkDigestAlgorithm>
-	<AndroidManifestMerger      Condition=" '$(AndroidManifestMerger)' == '' ">legacy</AndroidManifestMerger>
+	<AndroidManifestMerger      Condition=" '$(AndroidManifestMerger)' == '' ">manifestmerger.jar</AndroidManifestMerger>
 
 	<!-- Default Java heap size to 1GB (-Xmx1G) if not specified-->
 	<JavaMaximumHeapSize Condition=" '$(JavaMaximumHeapSize)' == '' ">1G</JavaMaximumHeapSize>