Animate clear all button's alpha after last task has scrolled beyond its edge.

Test: manual
Fix: 208879377
Change-Id: Iad43abbe36d11317869a0c5aa6a8d59fd1fddb67

quickstep/src/com/android/quickstep/views/ClearAllButton.java

@@ -141,7 +141,10 @@ public class ClearAllButton extends Button {
         }
         applyPrimaryTranslation();
         applySecondaryTranslation();
-        mScrollAlpha = 1 - shift / orientationSize;
+        float clearAllSpacing =
+                recentsView.getPageSpacing() + recentsView.getClearAllExtraPageSpacing();
+        clearAllSpacing = mIsRtl ? -clearAllSpacing : clearAllSpacing;
+        mScrollAlpha = Math.max((clearAllScroll + clearAllSpacing - scroll) / clearAllSpacing, 0);
         updateAlpha();
     }
 

quickstep/src/com/android/quickstep/views/RecentsView.java

@@ -1289,10 +1289,11 @@ public abstract class RecentsView<ACTIVITY_TYPE extends StatefulActivity<STATE_T
                     return;
                 }
                 TaskView taskView = getTaskViewAt(mNextPage);
-                // Only snap to fully visible focused task.
-                if (taskView == null
-                        || !taskView.isFocusedTask()
-                        || !isTaskViewFullyVisible(taskView)) {
+                // Snap to fully visible focused task and clear all button.
+                boolean shouldSnapToFocusedTask = taskView != null && taskView.isFocusedTask()
+                        && isTaskViewFullyVisible(taskView);
+                boolean shouldSnapToClearAll = mNextPage == indexOfChild(mClearAllButton);
+                if (!shouldSnapToFocusedTask && !shouldSnapToClearAll) {
                     return;
                 }
             }
@@ -4535,7 +4536,7 @@ public abstract class RecentsView<ACTIVITY_TYPE extends StatefulActivity<STATE_T
                 ? getClearAllExtraPageSpacing() : 0;
     }
 
-    private int getClearAllExtraPageSpacing() {
+    protected int getClearAllExtraPageSpacing() {
         return showAsGrid()
                 ? Math.max(mActivity.getDeviceProfile().overviewGridSideMargin - mPageSpacing, 0)
                 : 0;