[Xamarin.Android.Build.Tasks] ReOrder when Assets are Processed.

Context dotnet#1150

The way the current system works is that the `UpdateAndroidAssets`
target is called as part of the `ResolveReferencesDependsOn`. This
makes it impossible to do any kind of processing of assets after
the project has already built. It also menas that a straight up
call to `Build` will result in all the assets being copied into
the intermediate directory. If your project has a large number
of assets that is going to take time. Note this might also be
called as part of a designtime build at the moment... not good.

So is there a reason why we call `UpdateAndroidAssets` before
`Compile`? We don't generate any entries in the Designer.cs files
for them.. We don't clean them up.. we dont appear to do anything
except package them! So the anser looks to be a resounding NO!

So we shall move the `UpdateAndroidAssets` to be part of the
`_CreateBaseApkDependsOnTargets`. This means they will be
processed just before we create the base apk. This will
occur after compilation has completed. So we have a win win..

This commit also includes a unit test which does some post processing
after the `Compile` target to update an asset, it makes sure that
post processing does work.

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

@@ -12,6 +12,51 @@ namespace Xamarin.Android.Build.Tests
 	[Parallelizable (ParallelScope.Children)]
 	public class BuildAssetsTest : BaseTest
 	{
+		[Test]
+		public void CheckPostCompileAssetsIncludedInAPK ()
+		{
+			var path = Path.Combine ("temp", TestName);
+			var proj = new XamarinAndroidApplicationProject () {
+				ProjectName = "App1",
+				IsRelease = true,
+				OtherBuildItems = {
+					new AndroidItem.AndroidAsset ("Assets\\asset3.txt") {
+						TextContent = () => "PlaceHolder",
+						Encoding = Encoding.ASCII,
+					},
+					new AndroidItem.AndroidAsset ("Assets\\subfolder\\asset4.txt") {
+						TextContent = () => "Asset4",
+						Encoding = Encoding.ASCII,
+					},
+				},
+				Imports = {
+					new Import (() => "My.Test.target") {
+						TextContent = () => @"<Project xmlns=""http://schemas.microsoft.com/developer/msbuild/2003"">
+	<Target Name=""CustomTarget"" AfterTargets=""Compile"" >
+		<WriteLinesToFile
+			File=""Assets\\asset3.txt""
+			Lines=""Asset3""
+			Overwrite=""true""/>
+	</Target>
+</Project>"
+					},
+				},
+
+			};
+			using (var b = CreateApkBuilder (Path.Combine (path, proj.ProjectName))) {
+				Assert.IsTrue (b.Build (proj), "{0} should have built successfully.", proj.ProjectName);
+				using (var apk = ZipHelper.OpenZip (Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath, "android", "bin", "packaged_resources"))) {
+					var item = "assets/asset3.txt";
+					var data = ZipHelper.ReadFileFromZip (apk, item);
+					Assert.IsNotNull (data, "{0} should be in the apk.", item);
+					var text = Encoding.ASCII.GetString (data);
+					Assert.AreEqual ("Asset3\n", text, $"The Contents of {item} should be \"Asset3\" but was {text}");
+				}
+				Directory.Delete (Path.Combine (Root, path), recursive: true);
+			}
+
+		}
+
 		[Test]
 		public void CheckAssetsAreIncludedInAPK ()
 		{

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

@@ -536,7 +536,6 @@ Copyright (C) 2011-2012 Xamarin. All rights reserved.
 	</CoreResolveReferencesDependsOn>
 	<ResolveReferencesDependsOn>
 		$(CoreResolveReferencesDependsOn);
-		UpdateAndroidAssets;
 		UpdateAndroidInterfaceProxies;
 		UpdateAndroidResources;
 		$(ApplicationResolveReferencesDependsOn);
@@ -1907,6 +1906,7 @@ because xbuild doesn't support framework reference assemblies.
 		_CheckDuplicateJavaLibraries;
 		_GetAdditionalResourcesFromAssemblies;
 		_CreateAdditionalResourceCache;
+		UpdateAndroidAssets;
 		$(_AfterCreateBaseApkDependsOnTargets);
 	</_CreateBaseApkDependsOnTargets>
 	<_CreateBaseApkInputs>