Implement back navigation on communal hub

Navigating back on the hub will take the user back to the blank scene,
and close the hub.

Fixes: 346331399
Test: flashed and verified back gesture on right side of the hub
will close the hub, while left side doesn't trigger back gesture.
Flag: com.android.systemui.glanceable_hub_back_gesture

Change-Id: I86dc45a0c3fabd3c9f548bc21e18b1fc157ae3c5