Call ImageDecoder directly in ResourcesImpl

Test: Existing tests

Add a new (hidden) ImageDecoder.Source that accepts an AssetInputStream.
This allows us to create an AnimatedImageDrawable without fear of the
client closing the stream.

Call it from ResourcesImpl instead of Drawable.createFromResourceStream.

Change-Id: I07e00ca60c97538335a6310e830b73211fd8e7bb