avif: Add thumbnail support

Similar to HEIC, AVIF files can also carry a thumbnail image in
the container alongside the main image. Add support for parsing
and decoding it.

Test: New CTS test added in ThumbnailUtilsTest. Existing tests continue
to pass.
Bug: 141654151

Change-Id: Ifb07f6ed0386af852dbcd0d3554409975e5f601a