Navigation bar improvements (WIP)

The following things could be improved in navigation bar:
- if both side keys are set to empty and there is 4-button setup, set
  the side keys view to GONE
- if HOME key is displayed and RECENT key is hidden by the user, make
  holding HOME key
  and releasing it on the key display recents if held long enough
- if MENU key is always present in navigation bar, do not tell apps to
  show also a menu key (this is currently implemented via properties and
  requies SystemUI to run as system UID)
This will allow setting up the navigation bar keys as on they were on
older phones without the navigation bar and keep every functionality.

Change-Id: I6445818cd65ee856ff2fe7ef3db06358b2b34c5d