Split core window addition logic into addWindowInner()

This will make it easier to place closer creating and setting
ready a transition when a new window is added.

The actual logic for presentation windows will be handled in a
separate change.

Flag: EXEMPT refactoring
Bug: 390482707
Test: WmTests:WindowManagerServiceTests
Change-Id: Ibc123832d7491bf84e9a84e568d8a9baa78a3900