Hide heads up settings and show a notice if heads up is disabled.

This follows a UI pattern used at several other places, such as
daydream, screencast, profiles or blacklist settings.

Change-Id: I5635338ceccae8ba8450284d3078889333fb77c6
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
Merge changes I1c63fd26,I5635338c into pac-4.4

* changes:
  Settings: Fix switch state being out of sync.
  Hide heads up settings and show a notice if heads up is disabled.