Don't set relative z of window to IME if starting window

If a window is a starting window, it shouldn't get relative z to IME
since we don't want to change its z order.

Test: Split screen with pip and opening new app in split
Test: WindowStateTests#testNeedsRelativeLayeringToIme_startingWindow
Fixes: 158762203
Change-Id: I0be39285a96bc6458e077b772d0315be6075e0b0