Fix missing gesture insets for 3-button navigation

After the refactor of flexible insets, the provided insets of 3-button
nav bar doesn't match the old behavior. This patch brings back the
correct behavior.

That is: provide the nav bar frame as the bottom gesture insets, and
provide the side gesture insets if there're side cutouts.

Test: WindowInsetsPolicyTest
Bug: 263234979
Change-Id: I430cfd438c5ae1c8bd129b8703648e66efa65719