stagefright: MPEG4Writer stops the source before exiting the thread.

While pushBuffer is waiting for a condition that need to be signaled by the
writer thread, it is possible that the writer thread has already exited due to
error that happend during processing previous buffer. In this case, the muxer
thread will hang over this function.
So the writer thread needs to stop the source from feeding more buffer to it.

Bug: 26928205
Change-Id: Ifd35c29176fb817ed0f718f8f60a03bef6dea7ce