manager: hang up secondary calls

Fixes part of #209 by automatically rejecting secondary calls.

WIP: Because it's not tested. Also currently not sure if changing the state as is done here would trigger the "Missed Call" notification.

Also: We want to use the the "quiet" profile for ringing during an ongoing call. (see https://source.puri.sm/Librem5/calls/-/issues/209#note_142438)