Fix text extraction from notifications

Summary:
Fixes an error of extracting SpannableString as String (which resulted in notification text being null).
Fixes group conversation text extraction on API <28.
Also includes some minor refactoring.

Reviewers: nicolasfella

Reviewed By: nicolasfella

Subscribers: kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D22140