Refactor StageCoordinator to use SplitMultiDisplayHelper

In order to make StageCoordinator display aware, refactor it to use the
newly added hierarchy map keyed by displayId in SplitMultiDisplayHelper.

Flag: EXEMPT refactor
Bug: 404603801
Test: manual
Change-Id: I9517934cf032c3995ce1844c35d73d9a679bfd44
4 files changed