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 (reverted from commit fa000453dce8936ee6805c8d953f3ab8d42f2b21)