fix 3 bugs with immersivemode listpreference entries

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
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

Change-Id: I10d8a40eceb3a35c63964cfb95d4af84c12fce7e