Allow some app windows to display above the IME.

- Display child windows of the current IME target above the IME.
- Display app windows above the current IME target above the IME.

Regression in the functionality was introduce in
I6f8bf15ba246fac69c4a496ebb1d9e0b9b6a95a2 when we switch away from using
the window list for z-ordering and using the hierarchy which has the IME
above all app windows.

Change-Id: I399aab7fd5ad7327ef6bc29d48f6d9bf48a6ac6c
Fixes: 33128382
Test: bit FrameworksServicesTests:com.android.server.wm.WindowLayersControllerTests