Add VPX output buffer size check

and handle dead observers more gracefully

Bug: 27597103
Change-Id: Id7acb25d5ef69b197da15ec200a9e4f9e7b03518

bool OMX::CallbackDispatcherThread::threadLoop() {
 return mDispatcher->loop();
}

bool OMX::CallbackDispatcherThread::threadLoop() {
 return mDispatcher->loop();
}
