Break apart DISABLE_HOME and DISABLE_RECENT.

Additionally, start using setSystemUiVisibility() where
possible in the keyguard to allow activities and dialogs to
re-enable some of the navigation keys (notably: home but not
recents).

Finally, stop disabling MENU for activities atop the keyguard.

Bug: 5380495 // no home in driveabout, clock
Bug: 5396134 // able to show home/recent in keyguard
Change-Id: I04eb224554ee8cff79476b85148c4cda75bb0b62