Block all touches from reaching the hub when the shade is open

Touches on blank areas in the quick settings part of the shade seem to
fall through and reach the glanceable hub since we're in the same
window. To prevent this entirely, we just add a full-size blank Box
above the rest of our layout to block touches when needed.

Bug: 336351031
Fix: 336351031
Test: atest CommunalViewModelTest
Flag: ACONFIG com.android.systemui.communal_hub TEAMFOOD
Change-Id: I3c1419cb08bdf4bbba0eb9bb3af100b3b8dc3d5e