Add AMEDIAFORMAT key for AVIF thumbnail CSD

AVIF files can contain thumbnail images within a file. The codec
specific data for the thumbnail (AV1C Box) can be different than the
one for the main image. So we need a separate AMEDIAFORMAT_KEY to
carry that information.

This CL simply adds the key. Actual usage will come in a separate CL.

Bug: 141654151
Change-Id: I414c039f337f78ad71cde9bf2d0fb4a6e6d4bcdf