Enable back gesture on SUW for expressive theme

According to new spec, for devices that have a different theme, Setup
Wizard should no longer have the back button, and instead should use the
back gesture in between screens.

Fix: 349495243
Flag: EXEMPT bugfix
Test: Set the System property for
setup wizard device theme to be expressive. Rerun setup wizard and
ensure the back gesture works. For the existing theme, the
behavior should remain unchanged.

Change-Id: I26c7ae2b7c10a7dcce1c0556fb6169446600bee2