Introduced the visual stability manager

Notifications used to roam around and reorder whenever
they wished. Those little beasts must be tamed, hence
a new visual stability manager is introduced that
dictates the terms of their interplay.

Test: manual: add heads-up and see if they correctly appear
Test: runtest -x packages/SystemUI/tests/src/com/android/systemui/notification/VisualStabilityManagerTest.java
Bug: 33773401
Merged-In: I8d7596fa7c14e0df68459a77d445f618d517ad51
Change-Id: I8d7596fa7c14e0df68459a77d445f618d517ad51