HeadsUp: optional whitelist mode (1/2)

Sick of blacklisting every app except a few? So was @jimsth

Currently, this re-uses the same list, and doesn't autoclear whatever was there when it changes.
This may change at some point.

Change-Id: I0fd52f22120ff36e406a19eda6774fbdfcfca292
BaseStatusBar: remove empty ContentObserver.onChange override

Change-Id: I5d88f44dc61849e856cd62c688d1c6c8520e08dc