libstagefright: Fix crash due to race condition

Crash occurs due to race condition between ACodec
and MediaCodec updating buffer metadata, as ACodec
overrides "omxFlags" field updated by MediaCodec,
which asserts that the "omxFields" field be present
while processing the buffer.

Added change to serialize the metadata update by
ACodedc and MediaCodec

Change-Id: Ib555490233c2bb227d2ff604ed6b01c80717d2ce
CRs-Fixed: 717809
CRs-Fixed: 714606