DvrBufferQueue: Add basic event callbacks

Currently, there is no guarantee of thread safty and on which thread
those callbacks are going to be fired.

Three new DVR APIs introduced:
1/ dvrReadBufferQueueSetBufferAvailableCallback
2/ dvrReadBufferQueueSetBufferRemovedCallback
3/ dvrReadBufferQueueHandleEvents

Bug: 36748012
Test: buffer_hub_queue-test, dvr_api-test
Change-Id: I79900c9d205f1808b02400b379c79adb366a1104