Don't show both the IME Switcher and A11Y buttons

The IME Switcher shares the same spot with the accessibility button on
the navigation bar. In the previous implementation of the NavigationBar,
if the a11y button is visible, the IME Switcher button would not be.

With the recent unification of the navigation bar and the taskbar, both
buttons could be made visible at the same time. This fixes the taskbar
logic to only show the IME Switcher button when it should be visible and
the a11y button is not visible. This logic is only applied on phone
mode, the only case where the buttons overlap.

Flag: EXEMPT bugfix
Bug: 378803994
Test: switch to 3 button nav, show accessibility shortcut and dock it to
  the navigation bar, show the IME with at least two keyboard langauges,
  observe that only a11y button becomes visible
Change-Id: Ifc082d51e02d74f7dbf4516814121b67c0a6aa1a