This CL adds a layer of OnBackPressedHanlderRouter to Launcher:
1. 4 OnBackPressedHandler(s) are added in such order: auto cancel action mode handler, drag handler, view handler and state handler
2. first handler who can handle back will handle the entire back gesture
3. Let WidgetsFullSheet to handle widget to all apps transition
Bug: b/260956481
Test: manual
Change-Id: Idbce3dcec746226dd68aaabaddc8fe01334e9673
