Reduce API exposed through dbus

Summary:
Don't use QDBusConnection::ExportAllContents
No need to make connect and receivedPackage public slots (they're are the
parent's already)

Fixes T4975

Test Plan: Rough manual test

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Maniphest Tasks: T4975

Differential Revision: https://phabricator.kde.org/D3871