Fix back button sometimes not showing up in the initial setup screen

A few things to note
- We use gesture nav layoutter because it's the mode in which we show back button rendered by IME. There might be other historical reasons but it is what it is.
- The back button isn't necessarily laid out correctly with this layoutter (currently we don't adjust anything in layoutButtons.

Fixes: 338876161
Test: Repeatedly get in setup mode, and make sure the back button is visible
Change-Id: I402273c2868f3f46a12b00918dab3f36fbe82204