Don't allow screenshot swipe-dismiss on gesture-back regions

Currently we listen to touches in the gesture regions (the sides
of the screen) when determining whether a swipe has occurred. This
has the side effect of making it impossible to touch through the
screenshot overlay in those areas. This change makes it so we
ignore swipe-dismiss in those regions (but still listen for the
back gesture) and allow tapping through the overlay on the edges
of the screen.

Bug: 217571135
Fix: 217571135
Test: manual
Change-Id: I53869885c443320bd03cca80346741733f616629