Active display: allow hiding low priority notifications. [1/2]

Change-Id: I4831d6864b509e62b449c25619c59f5d6760ec47
Active display: Enable proximity sensor only when screen is off

This should ease people worried about cpu usage of the proximity
sensor while the screen is on.  The sensor listener will only be
registered when the screen goes off, and both active display and
pocket mode are enabled.  Once the screen turns on this listener
will be unregistered so no further updates are received when the
sensor values change.

Change-Id: I70979b5931815adb74bd8bc42e32886a6d829a12
Active display: Only fetch next notification if mNotification is null.

Change-Id: Ife8511d4a4b8c448bc7aa8997a250068bec5fae7
Active display: Don't show active display if call state is not idle.

This will prevent active display from displaying when a call is
coming in and let the call screen show up instead of active display.

Change-Id: I2cb79e097ae24dc6ad6c8a33be88eb19f7d8d235