Implement hub mode user activity timeout

When communal is showing, the user activity timeout will follow the
user's chosen screen timeout.

Fixes: 308845444
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT
Test: atest NotificationShadeWindowControllerImplTest
Test: changed screen timeout, opened communal hub, verified device
doesn't timeout after 10s and instead follows chosen screen timeout

Change-Id: Ie200fa135c8367f9d7d35128605602ae3b1a0825