stagefright: ExtendedStats: Add lock in TimeBoundVector

TimedBoundVector::clear() can get called from AwesomePlayer's
seek context whereas TimeBoundVector::add() is called from
logBitrate() in MediaSource::read(). Place a lock around all
TimeBoundVector APIs to avoid data corruption due to this
potential race condition.

CRs-Fixed: 753767
Change-Id: Ie70549d606515e2c0ccd8ac6bcbf07d1d5908f36