Clean up the preparing transition

Currently the preparing transition in some of the transitions
is cleaned up after the continueExpand signal is sent. With
bubble bar, the expand signal comes from launcher and it is
possible to swipe back while the animation is still running and
collapse the bubble bar. In that case the expand signal will
never be sent and the preparing transition will never get
cleared from the bubble. That would cause crashes the next time
we try to expand the bubble bar.

This change clears the preparing transition from the bubble as
part of the clean up step.

Bug: 415925442
Flag: com.android.wm.shell.enable_create_any_bubble
Test: atest BubbleTransitionsTest
Change-Id: Icd82ddbec6eaf322bf661eec84a9dbbb43373d0d