Fix alpha fading of NotificationShelf in LS

While we are on the LS and we close the Shade, the NotificationShelf
alpha was not animating. Now we share the same logic for calculating the
alpha values for the ExpandableNotificationRows and the
NotificationShelf.

Added new testCases to StackScrollAlgorithmTest to check an intermediate
state of the animated alpha values. Previously it was only checking the
end state, which is 0.0f.

Test: atest NotificationShelfTest
Test: atest StackScrollAlgorithmTest

Fixes: 237339523
Change-Id: I97b734d296397b6e2587512ff96ab62d976b6192