Fix race condition that skips corner updates on half swipe

Set mIsSwiping true before calling SwipeHelper#Callback#onBeginDrag;
otherwise onBeginDrag may use an outdated false and mistakenly include
the swiped view as part of a rounded section.

Fixes: 173753021
Bug: 171817112
Test: half swipe first and last notifs in section; corners always update
Change-Id: I115d30b10657b6edbf94d4d444058bdeeaf9000e

packages/SystemUI/src/com/android/systemui/SwipeHelper.java

@@ -327,8 +327,8 @@ public class SwipeHelper implements Gefingerpoken {
                     if (Math.abs(delta) > pagingTouchSlop
                             && Math.abs(delta) > Math.abs(deltaPerpendicular)) {
                         if (mCallback.canChildBeDragged(mTouchedView)) {
-                            mCallback.onBeginDrag(mTouchedView);
                             mIsSwiping = true;
+                            mCallback.onBeginDrag(mTouchedView);
                             mInitialTouchPos = getPos(ev);
                             mTranslation = getTranslation(mTouchedView);
                         }