Move assist handles from ScreenDecorations to navigation bar

To improve the performance of display cutouts, we need to first move the
assist handles out of ScreenDecorations so that we can remove the layer
of ScreenDecorations later.

Bug: 140781914
Test: atest ScreenDecorationsTest, atest AssistHandleBehaviorControllerTest
Change-Id: I7c7cf8e0590d92aa32758c2983fc49387d18f7ad
12 files changed