Centralize reorder-on-priority-change logic

The visual stability manager has a special exception that allows notifs
to be reordered if their priority changes. Previously, this was tracked
by various members and method spread across a lot of classes: it
was set in one place, read in another, and cleared out in a third. This
CL attemps to centralize this logic as much as possible to be just
contained in the VisualStabilityManager.

Test: atest, manual
Change-Id: I19e047711ef1f8bcb083bdd62f45f2fe68109cc4