Predictive back: widget to all apps

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

src/com/android/launcher3/allapps/DiscoveryBounce.java

@@ -81,10 +81,10 @@ public class DiscoveryBounce extends AbstractFloatingView {
     }
 
     @Override
-    public boolean onBackPressed() {
-        super.onBackPressed();
-        // Go back to the previous state (from a user's perspective this floating view isn't
-        // something to go back from).
+    public boolean canHandleBack() {
+        // Since DiscoveryBounce doesn't handle back, onBackInvoked() won't be called and we should
+        // close it without animation.
+        close(false);
         return false;
     }