Immediately stop bouncing transitions if the overscroll is empty

This CL immediately stops the overscrolling of a transition if the
overscroll spec is empty. This allows transitions to finish immediately
as soon as there won't be any visual effect during overscroll.

See b/327249191#comment10 for before/after videos.

Bug: 327249191
Test: In the STL demo app, select the VeryBouncy config and perform the
 Shade => Lockscreen/Shade transitions.
Flag: N/A
Change-Id: Ie5b5a93d7cbaebc647cb00f38455423332d8e395