Prevent notification swipes from getting canceled too soon.

The scroll view was grabbing the gesture as soon as its dy
crossed the scroller's own slop threshold. Once we detect a
horizontal swipe-to-clear we request that the scroller no
longer activate until the gesture is over.

Bug: 4998677
Change-Id: If916fc00d155516e0e4bfe8c2df19827a2f16fe6