Add support for consumer queue initial import and hangup.

- Add support for consumer queues to import buffers that are created
  before the consumer queue is created, making multi-consumer queue
  patterns possible. This is essential for VrFlinger operation.
- Add support for notifying consumer queues when the producer queue
  hangs up.
- Correct the epoll event loop to check for hangups even when buffers
  are available.
- Add method to retrieve the event fd from a queue.
- Add trace logging and minor cleanup.
- Improve bufferhubd dump state output.

Bug: 36401174
Test: build; bufferhub tests pass.
Change-Id: Idd6f38a3341c048192062734e288d11de48bc4d4