Use heif embedded thumbnail if available

Use the heif embedded thumbnail as long as it's not
too small to avoid decoding the full image and downscale.

bug: 74395267
Test: CTS MediaMetadataRetriever test; manual test of thumbnail
extraction by browsing new folders containing heif files in
Downloads app.
Change-Id: I5b2be0521452376153a773cb7671fefe7f9bc439