Block touches from descendants of an invisible panel.

This defends against a certain class of bugs where swiping
down the notification shade stops working because those gestures
are being sent to the shade contents rather than the panel
dragging code.

Bug: 178277858
Test: none; this adds robustness to broken states
Change-Id: Id088f20599cb272611c141948de6c6303b536960
1 file changed