Reduce duplicated layer assignment and placement

- LAYOUT_AND_ASSIGN_WINDOW_LAYERS_IF_NEEDED is done by
  INSETS_CHANGED.
- Wallpaper visibility is unrelated to layer change.
- Remove some legacy resizing code which are no longer needed
  since hierarchy merged.
- Remove unused PreAssignChildLayersCallback.

Bug: 159103089
Test: CtsWindowManagerDeviceTestCases

Change-Id: Ia2b14131c2d2c5ffc82702fdc653bdd7ee703ad4