Only add gesture insets to screenshot touch region in gesture mode

We prevent touches in the swipe gesture region from going through to
what's behind the screenshot UI so that we can recognize (and dismiss
on) the back gesture. However, in three-button mode in landscape the
'gesture region' includes the nav bar, so we were preventing taps from
going through to the nav bar. This change conditions on the device being
in gesture mode so that we ignore touches on the nav bar in 3-button
mode in landscape.

Bug: 372542780
Fix: 372542780
Flag: EXEMPT minor change restoring previous behavior
Test: manual (ensure taps on the nav bar in 3-button mode in landscape
are recognized)

Change-Id: I764e75dac7fc214a0136bea917ad865b5f2d40f8