Fix 5358124: Better transport control visibility management in lock screen

This changes TransportControlView to be "sticky" on lockscreen.  Basically, once it
appears on lockscreen, it stays there until unlocked and then locked again in paused state.

Tested basic design goals (using Music2):
- play then lock -> shows
- pause then lock -> not shown
- toggle pause to play while locked and not shown -> shows
- pause after played once while locked -> stays until we unlock and lock again while paused
- remote control play while paused & sleeping -> resume lockscreen -> shows

Also tested:
- configuration changes (orientation) to ensure widget continues to show after it once appears
- remote events while lock screen on -> keeps lockscreen on.
- remote events while sleeping -> doesn't wake.

Change-Id: I23418c5f7dfd1457c0844d2683772e8a3ed0abd1