IMS-VT: Clear progress spinner when primary call changes

This is a cherry-pick of partner CL in mm-wireless-dev.

In use-cases wherein UE receives waiting call or user tries to add call
soon after initiating a modify request, progress spinner is not hidden.
Fix this by clearing the progress spinner when primary call changes.

Change-Id: I3a5b28e36af3c73e797924cdf1b3cb87815296f1