Keep IME above starting window.

While app window is animating-in, IME window is child of aboveAppContainers.
If app requests IME on activity resume, IME will start animating-in
above Snapshot during app window animation.
However when app window animation finishes, IME's parent is no
longer aboveAppContainers, thereby making snapshot occlude
IME briefly. This results in flashing.

The fix is to keep IME in aboveAppContainers as long as startingWindow
is not removed from hierarchy.

Bug: 137534359
Test: Manual and atest ZOrderingTests AppWindowTokenTests
Change-Id: I78c939e8d1f23144162ba9eb8d834150fdd08ae8