Let launcher clean-up shell transition leashes

Originally disabled this when we were directly using
the transition's leashes for animation (since the transition
system owns those leashes). However, now that we create our
own leashes in RemoteAnimationAdapterCompat, we can use this
logic again.

Bug: 235616350
Test: launch and close app in quick succession
Change-Id: I27f70f26e114443aeb83671437f54747aae92c51