[Xamarin.Android.Build.Tasks] fix _CompileJava running too often (#2207)

Context: #2088
Context: #2129
Context: #2199

In 4deec52, I fixed the #deletebinobj problem we discovered.

However...

I introduced a regression to incremental builds, I noticed that the
`_CompileJava` target is now running on a second build with no
changes. Third build? oddly it gets skipped...

It seems to be due to our use of flag files:
1. `_UpdateAndroidResgen` updates `R.cs.flag` and uses the file as an
   output
2. `_GenerateJavaDesignerForComponent` now uses `R.cs.flag` as an
   input
3. `_GenerateJavaStubs` *also* updates the timestamp on `R.cs.flag`.
   This was added in 970da9e, as a workaround for our two instances
   of `ConvertResourcesCases`.
4. `_GenerateJavaDesignerForComponent` will now run again on the next
   build.

Since 1886e6f eliminated the second call to `ConvertResourcesCases`,
we don't need to update `R.cs.flag` in no. 3 any longer.

Removing the call to `<Touch />` `R.cs.flag` in `_GenerateJavaStubs`
fixed the issue, and I added some assertions in relevant tests to
check that the `_CompileJava` and `_GenerateJavaDesignerForComponent`
targets aren't running on incremental builds.

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

@@ -192,11 +192,19 @@ public void SwitchBetweenDesignTimeBuild ()
 				FileAssert.Exists (designtime_build_props, "designtime/build.props should exist after the second `Build`.");
 
 				//NOTE: none of these targets should run, since we have not actually changed anything!
-				Assert.IsTrue (b.Output.IsTargetSkipped ("_UpdateAndroidResgen"), "`_UpdateAndroidResgen` should be skipped!");
-				//TODO: We would like for this assertion to work, but the <Compile /> item group changes between DTB and regular builds
-				//      $(IntermediateOutputPath)designtime\Resource.designer.cs -> Resources\Resource.designer.cs
-				//      And so the built assembly changes between DTB and regular build, triggering `_LinkAssembliesNoShrink`
-				//Assert.IsTrue (b.Output.IsTargetSkipped ("_LinkAssembliesNoShrink"), "`_LinkAssembliesNoShrink` should be skipped!");
+				var targetsToBeSkipped = new [] {
+					//TODO: We would like for this assertion to work, but the <Compile /> item group changes between DTB and regular builds
+					//      $(IntermediateOutputPath)designtime\Resource.designer.cs -> Resources\Resource.designer.cs
+					//      And so the built assembly changes between DTB and regular build, triggering `_LinkAssembliesNoShrink`
+					//"_LinkAssembliesNoShrink",
+					"_UpdateAndroidResgen",
+					"_GenerateJavaDesignerForComponent",
+					"_BuildLibraryImportsCache",
+					"_CompileJava",
+				};
+				foreach (var targetName in targetsToBeSkipped) {
+					Assert.IsTrue (b.Output.IsTargetSkipped (targetName), $"`{targetName}` should be skipped!");
+				}
 
 				b.Target = "Clean";
 				Assert.IsTrue (b.Build (proj), "clean should have succeeded.");
@@ -318,7 +326,9 @@ public void CheckTimestamps ([Values (true, false)] bool isRelease)
 				var targetsToBeSkipped = new [] {
 					isRelease ? "_LinkAssembliesShrink" : "_LinkAssembliesNoShrink",
 					"_UpdateAndroidResgen",
+					"_GenerateJavaDesignerForComponent",
 					"_BuildLibraryImportsCache",
+					"_CompileJava",
 				};
 				foreach (var targetName in targetsToBeSkipped) {
 					Assert.IsTrue (b.Output.IsTargetSkipped (targetName), $"`{targetName}` should be skipped!");

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

@@ -2250,7 +2250,7 @@ because xbuild doesn't support framework reference assemblies.
 	ResourceDirectories="$(MonoAndroidResDirIntermediate);@(LibraryResourceDirectories)"
 	ResourceNameCaseMap="$(_AndroidResourceNameCaseMap)"
   />
-  <Touch Files="$(IntermediateOutputPath)_javastubs.stamp;$(_AndroidResgenFlagFile)" AlwaysCreate="True" />
+  <Touch Files="$(IntermediateOutputPath)_javastubs.stamp" AlwaysCreate="True" />
   <ItemGroup>
 	<FileWrites Include="$(IntermediateOutputPath)_javastubs.stamp" />
   </ItemGroup>