Reduce notification section gap on lockscreen; increase gap when going to shade

Move fractionToShade to AmbientState so it's accessible from both
StackScrollAlgorithm and NotificationShelf.

Fixes: 229112693
Test: StackScrollAlgorithmTest
Test: swipe down on lockscreen to go to shade
	=> section gap widens smoothly
Change-Id: I860d250986fd77643e67947e4b642e5bccf1dc29
7 files changed