Ensure only single pip dismiss magnetized target exists

- If init() is called multiple times, the previous magnetized targets
  can cause the wrong target position to be calculated when trying (or
  not trying) to dismiss

Bug: 213348722
Test: Manual
Change-Id: Ic1f972cc60eafca2302ad168065c974c8a0ee924

libs/WindowManager/Shell/src/com/android/wm/shell/pip/phone/PipDismissTargetHandler.java

@@ -140,6 +140,7 @@ public class PipDismissTargetHandler implements ViewTreeObserver.OnPreDrawListen
         });
 
         mMagnetizedPip = mMotionHelper.getMagnetizedPip();
+        mMagnetizedPip.clearAllTargets();
         mMagneticTarget = mMagnetizedPip.addTarget(mTargetView, 0);
         updateMagneticTargetSize();