Fix focusable on entire notification area if QS is expanded

Notifications are shown as shelf if QS is expanded. Because none
of notification contents are visible, the notification scroll
layout should be unscrollable and non-focusable.

Test: Use TalkBack to perform go/sysui-stackscroll-test
Change-Id: Icf27546fa43241b267be84761b7847a6e25c9a7a
Fixes: 72956614
1 file changed