Hiding split shade status bar for heads up notifications

Changing split shade status bar alpha so that it's dependent on shade expansion. This has two effects:
- it doesn't show on HUN (alpha = 0), which is the main issue
- it's animated on expansion and transition between launcher status bar and split shade status bar is less jarring - this likely needs to be adjusted in the future but for now should be good enough.

Also splitting getNotificationScrimAlpha into two separate ethods as it's usage got weirder over time and inline comments in method calls got outdated.
Also moving split of getNotificationScrimAlpha to seperate object - it feels a bit weird that this very specific interpolation calculation is in the general Interpolation object.

Bug: 198595419
Test: Receive HUN and NOT see split shade status bar on top of regular status bar
Change-Id: I3bfcd0697da4798fc845d2636303fd3616e131fc