muxer: set observer before we start waiting for read

If nobody ever reads the buffer before stop is called,
MediaAdapter will go ahead and release mCurrentMediaBuffer.
If it doesn't have an observer set, it will crash as it
does have a refcount of 1.

This can happen when the writer thread encountered an
error and calls stop early, in this case it will never
read the pending buffer.

bug: 70348810

Test:

- CTS MediaRecorderTest, MediaMuxerTest
- Hacked framework to produce a similar crash as in
  b/70348810 and verified no crash in stagefright.

Change-Id: Ia2017e6a13fca279c9603afe53ab6013b9c669f5