Allow AssetFileDescriptors to be used to create FRROs

Before this change, if a (system) app tried to use an image from its
assets to create an FRRO it would succeed but no image would display.
That is because when it is an asset we have an AssetFileDescriptor which
internally has a file descriptor, offset, and size and the file
descriptor would point at the apk itself. When this would get passed to
the native layer to create the frro only the file descriptor would get
passed so we would copy the whole apk, not just the image.

This allows apps to pass in an AssetFileDescriptor directly which we can
then use to also pass down the offset and size to the native layer and
only copy the image part.

Fixes: 289739832
Test: atest FabricatedOverlayTest
Change-Id: I80dceb19229a6a3c02851a5451ea6d46798981b3