Fix stale NSSL "fling" state caused by no-op fling

If a fling gesture ends with the current Y position in the exact end
position for the fling, then the code to run the fling animation is
bypassed, but the flag in AmbientState tracking whether a fling is
active is never reset. This CL replaces the short circuit logic with the
standard fling-end logic, ensuring that all expected code paths are
executed.

Fixes: 234824085
Fixes: 237272856
Test: manual
  1. Enter lockscreen
  2. Swipe up and then down to the exact same position, in one
     continuous gesture
  3. Wait for a notification update
  Observe: Shade resizes correctly to match the new contents
Change-Id: Ie8ddf1dbaaf1ba607657646db05d3eb622fc9415
Merged-In: Ie8ddf1dbaaf1ba607657646db05d3eb622fc9415