media: Support accurate pause/resume/stop in GraphicBufferSource.

Process all buffers that have been queued to the encoder on
StagefrightRecorder::pause()/stop(). Furthermore, add timestamp support for
pause/stop. Buffers willi keep getting encoded until 1) a buffer with a higher
timestamp is queued, or 2) the bufferqueue is abandoned by the producer.

Test: Recording with hacking GoogleCamera timestamp. And Mediarecorder CTS test.
Bug:32765698
Change-Id: I9ec33d635aef269836d7a5d9f4b906cb41b46a0d