GestureNav: Limit exclusion rects

Add limits for system gesture exclusion on the left and right edges.
This prevents non-immersive-sticky apps from totally disabling the
back gesture.

Bug: 135522625
Test: atest SystemGestureExclusionRectsTest

Change-Id: Ib26161663a6aababe803d3c70044f4017bdbe675
Exempt-From-Owner-Approval: Already +2ed on another branch