unhide API for synchronous audio capture.

unhide MediaSyncEvent class and AudioRecord.startRecording() method with synchronization
event allowing to start audio capture upon occurence of a specific event.
So far the only event defined is playback presentation complete but more events and synchronized
playback/capture methods could be defined in the future.

Change-Id: If05fc6cd125dd9afad451727f792a3fd9e916bef