[Xamarin.Android.Build.Tasks] _LinkAssembliesNoShrink timestamps (#2028)

Context: xamarin/monodroid#821
Context: 3d999d3#diff-42a9402e6466c65d49d0ee7caf21f327R164

Since commit 3d999d3, a test is failing downstream in monodroid that
is testing the following scenario:

 1. With `Fast Deployment` enabled, run `/t:Install`
 2. Make a small code change
 3. Run `/t:Install` again
 4. Assemblies are getting uploaded to the device that should already be
    up to date!

3d999d3 was addressing a symptom of the problem, but not the root
cause.  The `OutputStep` of the linker has various uses of
[`File.Copy()`][0], which preserves the timestamp of the source file:

> The attributes of the original file are retained in the copied file.

File timestamps are considered "attributes of the original file".

The solution, then, is to run the `<Touch/>` MSBuild task on the
linker's output.  This will create appropriate timestamps, and a
portion of the changes in 3d999d3 are no longer needed.

I updated the `CheckTimestamps()` test to validate `Debug` and
`Release` configurations, so the `_LinkAssembliesShrink` target is
also tested in this manner.  It turns out `_LinkAssembliesShrink` is
currently working fine, since it uses `$(_AndroidLinkFlag)` as its
`Outputs` Target attribute.

[0]: https://docs.microsoft.com/en-us/dotnet/api/system.io.file.copy?view=netframework-4.7.2#System_IO_File_Copy_System_String_System_String_

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

@@ -160,10 +160,8 @@ bool Execute (DirectoryAssemblyResolver res)
 							// We cannot just copy the linker output from *current* run output, because
 							// it always renew the assemblies, in *different* binary values, whereas
 							// the dll in the OptionalDestinationDirectory must retain old and unchanged.
-							if (File.Exists (assemblyDestination)) {
-								MonoAndroidHelper.SetLastAccessAndWriteTimeUtc (assemblyDestination, DateTime.UtcNow, Log);
+							if (File.Exists (assemblyDestination))
 								continue;
-							}
 						} else {
 							// Prefer fixup assemblies if exists, otherwise just copy the original.
 							copysrc = Path.Combine (OutputDirectory, filename);

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

@@ -205,11 +205,13 @@ public void TargetFrameworkMonikerAssemblyAttributesPath ()
 		}
 
 		[Test]
-		public void CheckTimestamps ()
+		public void CheckTimestamps ([Values (true, false)] bool isRelease)
 		{
 			var start = DateTime.UtcNow.AddSeconds (-1);
-			var proj = new XamarinAndroidApplicationProject ();
-			using (var b = CreateApkBuilder ("temp/CheckTimestamps")) {
+			var proj = new XamarinAndroidApplicationProject {
+				IsRelease = isRelease,
+			};
+			using (var b = CreateApkBuilder (Path.Combine ("temp", TestName))) {
 				//To be sure we are at a clean state, delete bin/obj
 				var intermediate = Path.Combine (Root, b.ProjectDirectory, proj.IntermediateOutputPath);
 				if (Directory.Exists (intermediate))
@@ -240,7 +242,8 @@ public void CheckTimestamps ()
 
 				//One last build with no changes
 				Assert.IsTrue (b.Build (proj), "third build should have succeeded.");
-				Assert.IsTrue (b.Output.IsTargetSkipped ("_LinkAssembliesNoShrink"), "`_LinkAssembliesNoShrink` should be skipped!");
+				string targetName = isRelease ? "_LinkAssembliesShrink" : "_LinkAssembliesNoShrink";
+				Assert.IsTrue (b.Output.IsTargetSkipped (targetName), $"`{targetName}` should be skipped!");
 			}
 		}
 

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

@@ -2078,7 +2078,13 @@ because xbuild doesn't support framework reference assemblies.
       LinkOnlyNewerThan="$(_AndroidLinkFlag)"
       ResolvedAssemblies="@(ResolvedAssemblies)" />
 
-	<!-- We don't have to depend on flag file for NoShrink, but it is used to check timestamp -->
+    <!--NOTE: the linker's use of File.Copy requires us to update the timestamps of output files -->
+    <ItemGroup>
+      <_FilesToTouch Include="$(MonoAndroidIntermediateAssemblyTempDir)*" />
+    </ItemGroup>
+    <Touch Files="@(_FilesToTouch)" />
+
+    <!-- We don't have to depend on flag file for NoShrink, but it is used to check timestamp -->
     <Touch Files="$(_AndroidLinkFlag)" AlwaysCreate="true" />
 </Target>