recorder: subscribe to audio/video sources

Makes the MediaRecorder an observer on AudioInput, AudioReceiveThread,
VideoInput and VideoReceiveThread, so they no longer need a reference
to the recorder.

Makes sure the framerate is set for the recorder by adding a way to get
the stream parameters from the video receiver and input, since AVFrame
does not carry framerate and time base information.

Adds way to detach the recorder from the media sources, so it can be
reattached on subsequent records.

Change-Id: I71bac0825541f6445861f4f949288e1c62416f05