[Xamarin.Android.Build.Tasks] Stop IncrementalClean removing assemblies. (#1255)

Fixes: https://github.com/xamarin/xamarin-android/issues/1164
Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=61108

The `IncrementalClean` target basically deletes a ton of stuff if
it's not in the `@(FileWrites)` group. Under certain conditons this
include assemblies in `$(OutDir)`. It mainly effects projects which
use a shared `$(OutDir)`.

So if a third level dependency is copied to the `$(OutDir)` but is
not referenced by the main app project we end up with the
`IncrementalClean` target deleting the file. This then breaks
packaging since we expect the file to exist.

This causes problems with TFS because it seems to ALWAYS use
a shared `$(OutDir)`.

To work around this problem we just need to tell the
`IncrementalClean` target to ignore any dll in the `$(OutDir)`. Since
we already have a target in place to handle this we can just expand
the list of files it should be ignoring.

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Common/ImportAfter/Xamarin.Android.Windows.targets

@@ -11,16 +11,16 @@ Copyright (C) 2014 Xamarin. All rights reserved.
 
 	<PropertyGroup>
 		<Debugger Condition="'$(Debugger)'==''">Xamarin</Debugger>
+		<_IsRunningXBuild Condition=" '$(MSBuildRuntimeVersion)' == '' ">true</_IsRunningXBuild>
 	</PropertyGroup>
 
-	<Target Name="_RegisterMdbFilesWithFileWrites" BeforeTargets="IncrementalClean">  
-		<CreateItem Include="$(OutDir)*.dll.mdb;$(MonoAndroidIntermediateAssemblyDir)*.dll.mdb;$(MonoAndroidIntermediateAssemblyDir)*.pdb;$(MonoAndroidLinkerInputDir)*.dll.mdb;$(MonoAndroidLinkerInputDir)*.pdb;$(_AndroidManagedResourceDesignerFile)">  
-			<Output TaskParameter="Include" ItemName="_FilesToRegister" />  
-		</CreateItem>  
+	<Target Name="_RegisterAndroidFilesWithFileWrites" BeforeTargets="IncrementalClean" Condition=" '$(_IsRunningXBuild)' == 'false' ">  
+		<CreateItem Include="$(OutDir)*.pdb;$(OutDir)*.dll;$(OutDir)*.dll.mdb;$(MonoAndroidIntermediateAssemblyDir)*.dll.mdb;$(MonoAndroidIntermediateAssemblyDir)*.pdb;$(MonoAndroidLinkerInputDir)*.dll.mdb;$(MonoAndroidLinkerInputDir)*.pdb;$(_AndroidManagedResourceDesignerFile)">  
+			<Output TaskParameter="Include" ItemName="_FilesToRegister" />
+		</CreateItem>
 		<CreateItem Include="$([System.IO.Path]::GetFullPath('%(_FilesToRegister.Identity)'))"
 				Condition="Exists('%(_FilesToRegister.Identity)')">  
 			<Output TaskParameter="Include" ItemName="_CleanCurrentFileWrites" />  
 		</CreateItem>
 	</Target>
-	
 </Project>