Clean up taskLaunch and taskLaunchCancel runnables more consistently

- Set both to null instead of just one or the other
- Clean up in TaskbarUIController#onDestroy instead of only in
  FallbackTaskbarUIController

Test: presubmit + leak detection
Flag: EXEMPT bug fix
Fixes: 411767825
Change-Id: I1571f126e2df01009777359eac1973976a8bb661

quickstep/src/com/android/launcher3/taskbar/FallbackTaskbarUIController.java

@@ -88,10 +88,6 @@ public class FallbackTaskbarUIController
     @Override
     protected void onDestroy() {
         super.onDestroy();
-        RecentsView recentsView = getRecentsView();
-        if (recentsView != null) {
-            recentsView.setTaskLaunchListener(null);
-        }
         mRecentsContainer.setTaskbarUIController(null);
         mRecentsContainer.getStateManager().removeStateListener(mStateListener);
     }