[bc25] Reset ShadeSessionStorage when Dual Shade is visible.

Bonus 1: This also slightly improves the definition of a "session" for the existing shade as well: not only when it's idle or transitioning out
(previous definition), but also when it's transitioning in.

Bonus 2: Refactor OverlayShade's combinePaddings() to be more readable
(albeit less concise).

Bug: 356596436
Flag: com.android.systemui.scene_container
Flag: com.android.systemui.dual_shade
Test: Existing unit tests still pass.
Change-Id: I068e8d92742f769f494376956462ec083477d02a