Add additional accessors to IonBuffer and BufferHubBuffer.

- Add accessor to underlying GraphicBuffer in IonBuffer.
- Add const accessors to underlying IonBuffer(s) in BufferHubBuffer.

Bug: 36401174
Test: bufferhub_tests; buffer_hub_queue-test
Change-Id: I24a6c19b141bb50ffa06e861a4533e10d55b5b20
2 files changed