Let position of system bars can be restored by swipe

In the new insets world, apps can move system bars while keeping them
visible, which can make the user difficult to access the bars. This CL
lets the user can restore the position of system bars by swipe.

This CL also removes some unnecessary logic and refactors some methods.

Fix: 152194798
Test: Manual test with NewInsetsMainActivity
Change-Id: I81c8a41ca88a403d291186d61ec597c6bc5d3d84