Hide status bar when glanceable hub is showing

This change introduces an alpha override when communal is showing to
prevent the status bar from becoming visible, for example, when dragging
down the shade.

This change is guarded by glanceable_hub_v2 being disabled. Once the
flag is enabled, status bar is visible on the hub and its behavior
should be identical to the lockscreen.

Test: atest KeyguardStatusBarViewController
Test: dragging down shade over hub doesn't cause status bar to reappear
Fix: 415141951
Flag: com.android.systemui.glanceable_hub_v2
Change-Id: I826e7cc47f27c4c223b5964b2e1223519597af9b