Loop ringtone in FindMyPhonePlugin

Summary:
BUG: 368089

FindMyPhone does not loop ringtones, which makes problems with short ringtones. Additionally, it depends on the volume of the alarm channel, so if the alarms are muted, no
sound can be heard. This patch raises the alarm volume to max when the activity gets started and restores the previous value when it stops.

Test Plan: Trigger findmyphone, hear sound looping. Set alarm volume to zero and check if it gets restored.

Reviewers: KDE Connect, mtijink

Reviewed By: KDE Connect, mtijink

Subscribers: mtijink, KDE Connect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D9947