Snap to relative position to snappd task when dismissing

- Calculate the diff to snapped page scroll and apply in onLayout, so tasks won't jump after dismiss when not in snapped position
- In grid, always keep the relateive snapped page unchanged to avoid jump

Bug: 188793333
Test: manual
Change-Id: Id11c2d700dc55440de39cc7409d06a712cedc9bc

src/com/android/launcher3/PagedView.java

@@ -104,6 +104,10 @@ public abstract class PagedView<T extends View & PageIndicator> extends ViewGrou
 
     @ViewDebug.ExportedProperty(category = "launcher")
     protected int mCurrentPage;
+    // Difference between current scroll position and mCurrentPage's page scroll. Used to maintain
+    // relative scroll position unchanged in updateCurrentPageScroll. Cleared when snapping to a
+    // page.
+    protected int mCurrentPageScrollDiff;
 
     @ViewDebug.ExportedProperty(category = "launcher")
     protected int mNextPage = INVALID_PAGE;
@@ -247,7 +251,7 @@ public abstract class PagedView<T extends View & PageIndicator> extends ViewGrou
         // If the current page is invalid, just reset the scroll position to zero
         int newPosition = 0;
         if (0 <= mCurrentPage && mCurrentPage < getPageCount()) {
-            newPosition = getScrollForPage(mCurrentPage);
+            newPosition = getScrollForPage(mCurrentPage) + mCurrentPageScrollDiff;
         }
         mOrientationHandler.set(this, VIEW_SCROLL_TO, newPosition);
         mScroller.startScroll(mScroller.getCurrX(), 0, newPosition - mScroller.getCurrX(), 0);
@@ -452,6 +456,7 @@ public abstract class PagedView<T extends View & PageIndicator> extends ViewGrou
      * to provide custom behavior during animation.
      */
     protected void onPageEndTransition() {
+        mCurrentPageScrollDiff = 0;
         AccessibilityManagerCompat.sendScrollFinishedEventToTest(getContext());
         AccessibilityManagerCompat.sendCustomAccessibilityEvent(getPageAt(mCurrentPage),
                 AccessibilityEvent.TYPE_VIEW_FOCUSED, null);