Add a delay before the transition leash is reparents after a launch.

This cleanup doesn't need to happen immediately, because its only
purpose is to avoid leaks. By adding a delay we can be reasonably
confident that it happens after the transition is done (i.e. after
the finish callback has been called), which avoids flickers.

Bug: 356065603
Flag: com.android.systemui.animation_library_delay_leash_cleanup
Test: atest ActivityTransitionAnimator + manual flicker check + Winscope leash cleanup check
Change-Id: I6974e73e782bbd204b4d196680258a40a03e7cc3