Fix inconsistency in nav mode changes

- Don't rely on the broadcast for determining overlay changes, the actual
  changes are propagated via asset path changes to the configuration
  only after which the correct resources can be read
- Remove unnecessary reloading of nav bar when changing modes, basically
  in the rare cases that the configuration change comes before the
  receiver what happens is that NavBarFragment is reloaded (because
  FragmentHostManager detects an interesting config change), but the
  NavigationModeController has the old value (because it hasn't received
  the broadcast), then when it receives the broadcast, it has to
  reinflate with the new mode.  Now that NavModeController is based on
  config changes (and FragmentService always posts config change updates)
  the NavModeController mode will always be updated before the fragment
  is reloaded (and NavBarInflaterView doesn't need to update on mode change
  at all)
- Remove some dead code

Bug: 156631944
Test: Change nav modes a bunch of times

Change-Id: I14ba57acab3683fcd187d0d4adcf87dc7e352b1d
Signed-off-by: Winson Chung <winsonc@google.com>