Clean up stale external transitions

If an external task view transition is started and returns a null
claim token, we now clean it up, otherwise it will remain in the
pending queue blocking subsequent transitions.

Bug: 392893178
Flag: com.android.wm.shell.enable_create_any_bubble
Test: TaskViewTransitionsTest
Change-Id: I50a8694a4694da237bd5bd1254725765b71f0ee5