Fix a couple issues with nav bar & rotation button

- Ensure that newly added buttons to the contextual group are not
  visible by default, this can cause issues where they are clickable
  if setButtonVisibility() is not called to ensure only one button
  is visible
- Fix an issue where the rotation controller and context group buttons
  were not updated on nav mode change (leading to floating rotation
  button w/ 3 button nav and rotation context button w/ gestural nav).
  This is a regression in S due to us not reinflating the entire nav
  bar view on nav mode change.
- Move ime switcher handling to NavBar to be consistent with other
  contextual buttons
- Prevent reloading nav bar multiple times on nav mode change

Fixes: 183475747
Test: Change nav modes and dump the context buttons to verify they
      are updated correctly
Test: atest SystemUITests
Change-Id: I023cf5f4a0689c6498043a6c8685c5c920d8da46