Always show ScreenDecorations when in gesture nav mode.

ScreenDecorations host the handles for the assist gesture, which are
available on non-rounded phones as well.

Only show the overlay that is actually in use.

Bug: 134487710
Test: atest ScreenDecorationsTest, manual testing on walleye
Change-Id: I00d1a274d5a236fb2f4679d80bd38db6f24d06bc