Dismiss RTT request dialog if RTT is already active.

On VZW, RTT request will be accepted by modem automatically, in which case the
dialog will be useless and should be dismissed. User won't see the dialog in
this case since it's not visible yet when dismissed.

This is a workaround since change in modem/telecom is not feasible at this point.

Bug: 80259700
Test: manual
PiperOrigin-RevId: 199334017
Change-Id: Ie5fb40e7a11975a423135ad7e5e6ef2dddbac56b