[rbp/omxplayer] Fix audio volume jumps when switching tracks

When the global volume has been set low, and a sequence of tracks are being played, there are complaints of occasional jumps to full volume.
This is down to a race condition where the volume request can arrive at OMXAudio before it has been initialised.
The fix is simple, don't send the volume change until m_CurrentAudio.started.
Merge pull request #2641 from popcornmix/fix_volume_jump

[rbp/omxplayer] Fix audio volume jumps when switching tracks