Buttons: Allow disabling hard navigation keys

This adds support for enabling the navbar on devices with
hardware keys.
This requires that the device supports the KeyDisabler
HAF methods, and is available in the Buttons menu

Change-Id: I01bd83b312745b4a8aa3bde0cf5f02ea99a888cc
Merge pull request #3 from CyanogenMod/cm-12.0

Buttons: Allow disabling hard navigation keys