Fix screenshot back behavior in 3-button mode

We weren't dismissing when the back button was pressed, because the
screenshot UI loses focus on touch_outside and so we never get the key
press. This change checks if the touch is in the navigation bar system
region, and doesn't reset the focus if so.

Bug: 441745433
Test: manual (using 3-button mode)
Flag: EXEMPT bugfix
Change-Id: I5ca573c43a1004e36632b5d868b57f686d4ab09c