Simplify NotificationScrollViewModel.expandFraction

The only time we actually want to expand the stack is when we're going
from Gone to Shade or Gone to QuickSettings, and we keep running into
bugs where expandFraction accidentally dropped to 0f and the stack
disappeared.

So, rewrite the definition of expandFraction to hardcode the expansion
for most scenes and transitions, and to use the upstream shadeExpansion
(and qsExpansion) only when needed.

Bug: 347769238
Bug: 347762707
Flag: com.android.systemui.scene_container
Test: manual: examine various scene transitions involving the stack
Change-Id: I02fe1a1558614bbf45b21cde06d076dcc9b259b1