Show back and IME switcher for gesture nav (1/2)

This shows back and IME switcher when in app taskbar
and IME is visible.
This doesn't remove the system bar just yet (will
show overlap). Next CL will remove the system IME buttons
so only launcher IME buttons show.

Bug: 191612881
Test: Used IME in gesture + 3 button with taskbar.
Change-Id: If39382c4d01f26a9350f7460d9e769ca9b57828c