heif: support heif image sequence and dual-function files

- Add a few methods to MediaMetadataRetriever to get still
  images and frames by index.

- Declare keys in MediaFormat related to heif still images.

bug: 63633199

test:
cts-tradefed run cts-dev --module CtsMediaTestCases --compatibility:module-arg CtsMediaTestCases:include-annotation:android.platform.test.annotations.RequiresDevice

Change-Id: I9556b27f7395a163aadd01e979930061878994d0