Allow choosing a ringtone for FindMyPhone

Summary:
BUG: 379023

Let users choose a ring tone in the settings.

Test Plan: Open settings, choose a ringtone, search for device.

Reviewers: #kde_connect, albertvaka

Reviewed By: #kde_connect, albertvaka

Subscribers: mtijink, andyholmes, albertvaka, apol, #kde_connect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D10332