[WIP] HeadsUp: optional whitelist mode (2/2)

The frameworks half of this is happy without this for now.
TODO: make the "whitelist" (known as the blacklist) reload properly when you open headsup settings after checking the whitelist mode checkbox?
      or maybe just try take a look at this tomorrow and facepalm about whatever stupid problem is impeding functionality

Flips the per-app HeadsUp logic

Change-Id: Ia8e32697ac668f413b589aeadb173551eb6ebcf9
Settings: Fix switch state being out of sync.

    - Since QS Tile can also set state of Heads up now,
    we have to set a content observer for the action bar
    switch.

Change-Id: I1c63fd2627a0f823eb6982456e1ed79ab84d1957