Clean up navigation settings

Aosp navigation and NX are independently accessible from a
common navigation fragment. Bar mode is selected from a
list preference. The settings for each mode is accessible
from a single preference, in which the target fragment changes
based on the mode. Clean up various strings and titles.

PS2: Move force navbar checkbox into KeySettings, where it
     should be along with KeyDisabler. Enable/disable
     navigation settings in Interface settings accordingly

PS3: Update hardkey button prefs if hardkeys are present,
     not if force bar is enabled/disabled

Change-Id: I125a2fb6bfd895e227ba48ea57389cf779201b74