heif: add muxer support for adding still images

This change adds basic support for adding still images to a
heif container.

bug: 63633199

test:
Locally built test app that adds more than one still images
and video sequences to a heif container. Verified the muxed
file can be played by locally built heif viewer app that
uses MediaExtractor/MediaMetadataRetriever/MediaPlayer apis.

Tested media post-submit CTS on Pixel2XL.

Change-Id: I185bbbbed3dbb4e76d64bcf7323ee44a42cc8623