Refactor enableBluetooth function

Summary: The code has quite some boilerplate and high levels of indentation, resulting in poor readability. Factoring out the DBus boilerplate makes it easier to focus on the essential parts.

Test Plan: (de)activated flight mode. Bluetooth was (dis)abled accordingly

Reviewers: Plasma, jgrulich

Reviewed By: jgrulich

Subscribers: plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D28714