heif: fixes for image sequences and dual-function files

Adding support for two new sets of APIs on MediaMetadataRetriever:

- getImageAtIndex() and getPrimaryImage()
- getFrameAtIndex() and getFramesAtIndex()

Outline of changes:

- Proper indexing of all displayable still images, so that they
  can be retrieved by getImageAtIndex()

- Exposing still images as "image/x.android.heic" tracks in
  MediaExtractor with necessary descriptive keys (such as "grid-*")

- Support to retrieve video frames by absolute index instead
  of timestamps, as image use cases mostly are interested in
  getting the images and care less about timing.

- Support to retrieve video frames in batches because retrieving
  one frame at a time is inefficient.

- Refactor image / frame decoding code into FrameDecoder, and split
  still image decoding and video sequence decoding into to sub
  classes to facilite future development.

bug: 63633199

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

Change-Id: I2fe8519fb6907f315a8b513921fc1cc7f436e28d