Fix for expanded view drop target was not shown.

Added a call to make the layer view visible during the drag over the
bubble bar event.

Bug: 396476697
Flag: com.android.wm.shell.enable_create_any_bubble
Test: Manual. Start with no bubbles in the bubble bar and drag an
application icon. Observe that the expanded view drop target is now
present.

Change-Id: I3a134a8bd7b8b2c7453c72fef08d990945a65f63