fix 4 bugs with immersivemode listpreference entries

another one that got stomped from resetting to the powermenu tile before fixing the NPE

1. devices with a navigation bar were not able to choose 'navbar only' because the setting
  to enable the navigation bar was never available to them (hammerhead tested)
2. setImmersiveModeEntries() was not updating on back key events from the navbar settings
3. setImmersiveModeEntries() does not need to be processed unless there's hardware keys
4. setImmersiveModeEntries() weren't updating properly on hardware only devices because of
  a flipped boolean (d2lte tested)

Change-Id: I40a3dd67948459637de98e23f37b1cea4fce6257