Prevent showing system IME buttons for gesture nav in taskbar (2/2)

Don't update visibility of back and ime switcher buttons
on sysui navbar, let launcher handle it via existing
sysui state flags.

Bug: 191612881
Test: Double IME buttons don't show on IME w/ taskbar
Change-Id: I689626cb4c923ce4092ad409ac6d99030af8c464