BaseStatusBar: performance update

don't register pie's content observers and broadcast receivers if the statusbar is recreated
 unless pie is actually enabled.  Since we pkill systemUI every time pie is toggled.. we can
just check if it's enabled when the statusbar is created and register them accordingly.

ps2: unobserve when pertinent

Change-Id: I2b91604e534794196846c8cecb55278f85d326eb
Merge "BaseStatusBar: performance update" into jb42