Assign animation layer above highest animation target.

Rather than above all stacks. This fixes the case where when
transitioning to the minimized home state, the secondary split
screen app and home are in a transition, but this transition needs
to occur under the primary split-screen app which is not a part
of any transition.

Bug: 69553456
Test: Manual. go/wm-smoke.
Change-Id: I3069e4a1bc3a30a1567eb51731c44f927cc0cefc