Remove back button and enable back gesture on SUW

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 button doesn't show up and the back gesture works. For the existing theme, the behavior should remain unchanged.

Change-Id: I6b8e95f4eeecdf7422c9f1c099b30a4fc2d2fd12