Clean up NotificationScrollViewModel.expandFraction

It got kind of tangly and squished with all the (necessary) nested
logic, so split some things out into helper functions.

Bug: 347769238
Flag: com.android.systemui.scene_container
Test: manual: examine various scene transitions involving the stack
Change-Id: I02fe1a1558614bbf45621cde06d076dcc9b259b1