Make the task remove a wct

When dragging to dismiss bubbles while they're expanded there's a
bit of a hiccup when there's another bubble to select, removing the
task via a transition resolves this

Test: manual (with other CL):
    - have 5 bubbles, expand the stack, drag each bubble to the
      dismiss target
    => observe that the animation to select the next bubble when the
       current one is selected is smooth
Bug: 279074302
Change-Id: I9989112a4834c99ae4bfa5cee6f3bf60270197dd