Fix frozen notifications after bouncer dismissal

Distinguish between two types of fling-down on keyguard:
1) the kind after a swipe-up that does not show bouncher
	=> keep stack height the same
2) the kind after showing bouncer
	=> update stack height

Move NSSL#mIsFlinging to AmbientState

Bug: 219883571
Test: unlock hint, swipe up, show/hide bouncer => no stack freeze
Change-Id: I7af1a4f941d009f05e3fc9dc667a035d19e87d62
(cherry picked from commit 65da9766)
Merged-In: I7af1a4f941d009f05e3fc9dc667a035d19e87d62