Add windows to window tokens in expected z-order

Decouple the logic for adding window to a position in the parent
window token from the position we are adding the window to in the
window list. The window token now adds the windows in order based
on the rules the rest of the system is using which makes the code
a little more straightforward to follow.

Test: bit FrameworksServicesTests:com.android.server.wm.AppWindowTokenTests
Change-Id: Ic9b724fba02279a0c4e92508d39e5e35171b6d8d