Do not use prefixOrderIndex for animation layer

We may set a z-order different from the prefix order, for example
TaskDisplayArea#adjustRootTaskLayer. As a result, we should no longer
set animation leash layer based on the prefix order.

Fix: 208793409
Test: manually test launching activity over another
Change-Id: If8d545ca7a9ac42162262ad3a8f95b4ac9f65585