DO NOT MERGE mm-video-v4l2: venc: Avoid processing ETBs/FTBs in invalid states

(per the spec) ETB/FTB should not be handled in states other than
Executing, Paused and Idle. This avoids accessing invalid buffers.
Also add a lock to protect the private-buffers from being deleted
while accessing from another thread.

Bug: 27903498
Security Vulnerability - Heap Use-After-Free and Possible LPE in
MediaServer (libOmxVenc problem #3)

Change-Id: I898b42034c0add621d4f9d8e02ca0ed4403d4fd3