Migrate from PreferenceActivity to PreferenceFragment because it has been deprecated since Android 3.0 API11

Summary:
PreferenceActivity has been deprecated since Android 3.0 API 11 in favour of PreferenceFragment

| {F6515113} | {F6515114}|
|Before|
| {F6515111} | {F6515112}|
|After|

Test Plan:
Apply patch and observe that the DeviceSettings and PluginSettings screen behave as expected
and now also follow material design guidelines

Reviewers: #kde_connect, nicolasfella

Reviewed By: #kde_connect, nicolasfella

Subscribers: kdeconnect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D17859