[Lock screen] Follow KDE title & subtitle style

Summary:
We're currently using a custom tyle for song titles and artist info in the lock screen's media controls.

This patch applies common KDE title & subtitle style as seen in e.g. Kickoff or plasma-nm applet-

Because the lock screen has increased font sizes, corrections for the title and subtitle have also been made.

Test Plan:
Before:
{F6960714}

After:
{F6960717}

Reviewers: #plasma, #vdg, ngraham

Reviewed By: #vdg, ngraham

Subscribers: broulik, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22231