Report gesture height for mandatory system gestures

- We are currently reporting the content insets for the mandatory system
  gestures in taskbar, but for button nav we should actually be
  reporting zero, and for gesture nav we should either report the
  content height if the taskbar is pinned, or the gesture height
  otherwise as the bottom mandatory gesture inset

Fixes: 340134342
Test: atest android.systemui.cts.WindowInsetsBehaviorTests
Change-Id: Ie4d56b62c903c273db95c19c9d34fcfe6c9ce486
Merged-In: Ie4d56b62c903c273db95c19c9d34fcfe6c9ce486
(cherry picked from commit 05682a05)