[Flexiglass] Prevent touches with brightness mirror

When the brightness mirror is being shown (dragging brightness), we set
the alpha to the rest of the components in the shade to 0. We should
also prevent any touches.

Test: manual, drag brightness and press in tiles or notifications (split
shade).
Fixes: 441102725
Flag: com.android.systemui.scene_container

Change-Id: I0e15ab8a3fbe4f2d44986204090ee4653439454a