Monitor screenshot shelf view for outside touch events

We need to release focus on the screenshot window when we get outside
touches to allow the keyboard to come out (e.g. when taking a screenshot
on lockscreen).

Flag: com.android.systemui.screenshot_shelf_ui2
Bug: 329659738
Bug: 339591708
Fix: 339591708
Test: set screen lock to password, verify that taking a screenshot on
lockscreen and tapping the preview allows the keyboard to come up for
unlock

Change-Id: Icce3c28351382d65c24343cb60dd4103242dfccc