Round adjacent corners when swiping notification

Unround corners instantly when notification starts to snap back,
which feels more responsive.

Fix bug where fast consecutive swipes are ignored because
we did not set mTouchedView to the latest MotionEvent's view.

Bug: 171817112
Test: swiped sandwiched notification
     => rounds bottom corners of notification before
     => rounds top corners of notification after
        (in the same section; other sections not affected)
Test: swipe multiple notifications quickly
     => no jank

Change-Id: I1a808dd92f9e46df4767681d861ea73530d732a8