[Xamarin.Android.Build.Tasks] Ignore AndroidAsset items which are folders.

[dellis1972]

When adding a new subfolder in the IDE (VSForMac) we end up
with a folder which has a build action of AndroidAsset.
This causes the copy task to fail with

Cannot copy <folder> to <destfolder>, as the source file doesn't exist",

This is because we assume that all items in AndroidAsset (and
AndroidResource) and files. It looks like this is not the case.
While a work around is easy .. simply remove the item. It does
require that the user manually update the csproj.

So this commit uses Directory.Exists to check to see if the
item is a directory or not. If it is we skip over it. Otherwise
its a file so we should copy.

We do this in Files.cs but also in AndroidUpdateResDir.cs.
The latter has been changed to output a list of the "Resolved"
files which will not include any folders.

[dellis1972]

build

[jonpryor]

The cause of the PR build break:

…/xamarin-android/tests/BCL-Tests/Xamarin.Android.Bcl-Tests/Xamarin.Android.Bcl-Tests.csproj (SignAndroidPackage) ->
…/xamarin-android/bin/Debug/lib/xamarin.android/xbuild/Xamarin/Android/Xamarin.Android.Common.targets (_BuildApkEmbed target) ->
	…/xamarin-android/bin/Debug/lib/xamarin.android/xbuild/Xamarin/Android/Xamarin.Android.Common.targets:
error : Error executing task BuildApk: Could not find file "…/xamarin-android/bin/Debug/lib/xamarin.android/xbuild/Xamarin/Android/lib/arm64-v8a/libmono-android.debug.so"

is because the new Xamarin.Android.Bcl-Tests.csproj explicitly sets $(AndroidSupportedAbis) to include all ABIs.

(Compare to e.g. d15-6.)

@dellis1972: Please update Xamarin.Android.Bcl-Tests.csproj so that it only requires armeabi-v7a;x86, so that the PR builds can build. :-)

[dellis1972]

build

[dellis1972]

yay green builds :)