Fix IME nav bar button flickers

This fixes the back navigation button flicker in 3-button navigation
when switching IMEs, by setting it to the default (non-IME) position
when the IME switcher dialog is shown.

This also fixes the IME switcher button flicker in 3-button and gesture
navigation modes when switching IMEs.

Test: install two IMEs, open the IME and have the IME switcher icon
  visible, test switching IMEs in 3-button and gesture navigation mode
  and observe the behaviour of the navigation buttons.
Bug: 287045025
Change-Id: I9c0d521d372db8b33f4898eb25bdef1e6fc50e48