Added getLastAcquiredFrameNum

This will allow VRI to ask BBQ what buffer was actually acquired on the
last draw.

Test: blast sync
Bug: 195262673
Bug: 193634619
Change-Id: I492651e8e6d333ef11b682cec939d81057ae197d
Merged-In: I492651e8e6d333ef11b682cec939d81057ae197d
2 files changed