Low battery warning policy (2/2)

Option to choose how low battery warning will be indicated
- Popup dialog and sound
- Popup dialog only
- Sound only
- Off

Change-Id: I6fa2c45301d37ca3086703bf1eddde12b45ba6f9
RB Settings: Do not show some options for Nexus devices.

Since Nexus devices do not have hardware keys, it is useless to display
Hardware keys options in RootBox Settings.