[Xamarin.Android.Build.Tasks] simplify inputs to _CompileToDalvik

Recently I've been discovering more and more places where usage of
`**\*.*` can hurt the performance of our build.

In this case, the `_FindCompiledJavaFiles` target:

    <Target Name="_FindCompiledJavaFiles" DependsOnTargets="_CompileJava">
        <CreateItem Include="$(IntermediateOutputPath)android\bin\classes\\**\*.class">
            <Output TaskParameter="Include" ItemName="_CompiledJavaFiles" />
        </CreateItem>
    </Target>

This target is running even in builds with no changes, so we are
*always* recursing directories and finding `*.class` files.

But since we are using a `classes.zip` file now, we don't need to
recurse and find the `*.class` files at all anymore!

I was able to remove this target completely, and change
`$(_CompileToDalvikInputs)` to use
`$(IntermediateOutputPath)android\bin\classes.zip`. Less inputs also
help build times, because MSBuild won't have to evaluate timestamps on
all the `*.class` files. It can just look at the single `classes.zip`
file.

To see the difference, a build with no changes was taking:

    51 ms  _FindCompiledJavaFiles                     1 calls
    30 ms  _CompileToDalvikWithDx                     1 calls
    Total time: 3.524s

After these changes:

    2 ms  _CompileToDalvikWithDx                     1 calls
    Total time: 2.386s

`_FindCompiledJavaFiles` is gone completely.

You can see there is some time savings in general here, I'm not sure
how MSBuild logs the time it is taking to compute `Inputs` and
`Outputs` of targets.

Author: jonathanpeppers

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

@@ -2470,16 +2470,8 @@ because xbuild doesn't support framework reference assemblies.
   </CreateItem>
 </Target>
 
-<Target Name="_FindCompiledJavaFiles" DependsOnTargets="_CompileJava">
-  <CreateItem
-    Include="$(IntermediateOutputPath)android\bin\classes\\**\*.class">
-    <Output TaskParameter="Include" ItemName="_CompiledJavaFiles" />
-  </CreateItem>
-</Target>
-
 <PropertyGroup>
 	<_CompileToDalvikDependsOnTargets>
-		_FindCompiledJavaFiles;
 		_GetMonoPlatformJarPath;
 		_GetAdditionalResourcesFromAssemblies;
 		_CreateAdditionalResourceCache;
@@ -2489,7 +2481,7 @@ because xbuild doesn't support framework reference assemblies.
 		$(MSBuildAllProjects)
 		;@(_JavaLibrariesToCompileForAppDx)
 		;@(AndroidExternalJavaLibrary)
-		;@(_CompiledJavaFiles)
+		;$(IntermediateOutputPath)android\bin\classes.zip
 		;@(ProguardConfiguration)
 		;@(MultiDexMainDexList)
 		;$(_AndroidBuildPropertiesCache)