Fix bubble bar drop target visibility.

The drop target view remained invisible after a drag operation because
its state was not cleared. This change adds logic to clear the state
when the drag concludes.

A test was also added to verify the fix.

Fixes: 427246053
Test: DragToBubblesZoneChangeListenerTest
Flag: com.android.wm.shell.enable_create_any_bubble
Change-Id: If4bef29d47bda113b2ebc173832a03c3b43fd69f