heif: add option for specifying bitmap pixel format

Add an option similar to BitmapFactory.Options to the bitmap
extraction APIs added in P to allow the app to specify bitmap's
pixel format. MediaMetadataRetriever's old getFrameAtTime()
only allows extraction in RGB565, for image use case the bitdepth
could be too low.

Also change return type of getFramesAtIndex to List as
Lint is complaining about returning raw arrays.

bug: 63633199
bug: 73886998

Change-Id: I40f0a421c767483e32c7744180dc5a187681e066