[monodroid] Fix builds when $(AndroidSupportedAbis) contains host

Commit 0c073f6 broke the src/monodroid build:

	src/monodroid/monodroid.targets: error : Cannot copy
	.../xamarin-android/src/monodroid/obj/local/host-Darwin/libmonodroid.so to
	.../xamarin-android/bin/Debug/lib/xbuild/Xamarin/Android/lib/host-Darwin/libmono-android.debug.d.so,
	as the source file doesn't exist.

This is because with 0c073f6, `$(AndroidSupportedAbis)` now contains
e.g. host-Darwin, which isn't a valid ABI name as far as `ndk-build`
is concerned.

Fix the src/monodroid build by excluding non-Android values.

Author: jonpryor

src/monodroid/monodroid.projitems

@@ -4,7 +4,7 @@
     <_SupportedAbis>$(AndroidSupportedAbis.Replace(':', ';'))</_SupportedAbis>
   </PropertyGroup>
   <ItemGroup>
-    <_MonoRuntime Include="$(_SupportedAbis)" />
+    <_MonoRuntime Include="$(_SupportedAbis)" Exclude="host-$(HostOS);host-win64" />
   </ItemGroup>
   <ItemGroup>
     <_RequiredProgram Include="xxd" />

src/monodroid/monodroid.targets

@@ -13,7 +13,7 @@
       Outputs="@(_MonoRuntime->'$(OutputPath)\%(Identity)\libmono-android.$(_Conf).so')">
     <Which Program="%(_RequiredProgram.Identity)" Required="True" />
     <PropertyGroup>
-      <_AppAbi>$(AndroidSupportedAbis.Replace(':', ' '))</_AppAbi>
+      <_AppAbi>@(_MonoRuntime->'%(Identity)', ' ')</_AppAbi>
     </PropertyGroup>
     <WriteLinesToFile
         File="jni\Application.mk"