Fix BufferHub state machine to return errors on invalid transitions.

The consumer side of the BufferHub flow did not have an adequate
state machine to track transitions and return errors when Acquire
or Release are requested from the wrong state. This bug allowed
other buggy usage to go unnoticed.

- Fix the consumer state machine to correctly validate all requests.
- Add tests to verify correctness of the fix.

Tested BufferHub with the new test before and after the fix and
verified that the test catches the problem and that the fix solves
the problem.

Bug: 62886596
Bug: 63149525
Test: bufferhub_tests passes.
Change-Id: I802679ed74c7f505b9243ba4048824350d4e37be