Cancel dark animation when new one arrives

Not doing so causes weird artifacts when power button is pressed
multiple times in a row and StackScroller would have the wrong
state if unlocked with fingerprint while animation is still running.

Test: lock from lock screen, fp before animation ends
Test: press power multiple times on lock screen, look at smooth animation
Change-Id: I381bf367533983595cd22ff080f51465f6c0f750
Fixes: 79747888