Always report mandatory insets for button nav

Fixes: 340134342
Fixes: 343817842
Test: atest android.systemui.cts.WindowInsetsBehaviorTests
Test: Verify mandatory insets vs window bounds in various
      orientations when folded/unfolded
Change-Id: I18d4f5d212d16c62d15d84b4842b3d20c0d8c5ae
Merged-In: I18d4f5d212d16c62d15d84b4842b3d20c0d8c5ae
(cherry picked from commit 762b3b46)