[modem-daemon] Add inhibition in logind when call is active

Otherwise if powerdevil is configured to suspend device after few minutes,
it will happily put it to suspend, freezing calls.

Fix regression after porting from oFono to ModemManager

Contributes to: #56
Replaces: !91



Co-Authored-By: Marco Mattiolo <marco.mattiolo@hotmail.it>