Display back button on home settings.

This change provides a back affordance on the Home settings when
accessed from Hub mode settings.

Bug: 263403020
Fix: 263403020
Test: Settings > Hub mode > At a glance, see back affordance
Test: long press launcher > Home settings, no back affordance
Change-Id: I96449d7eae4afac5ec3f61e5aa5a4f2c05b8cd51