[rbp/omxplayer] Remove clock manipulation

Currently the clock's state gets modified by OMXPlayer, OMXPlayerAudio and OMXPlayerVideo. This can lead to race conditions.

This change makes OMXPlayer the only place to modify the OMXClock state.
The previous change to move buffering to OMXPlayer level means the clock is paused whenever flushing or initialising audio/video components, so the locking and stopping of clocks from audio/video components is not required.

This PR includes the commits from #2926, and should be applied after it.