Add runWithPlugin helper

Summary:
There is a common pattern:
BackgroundService.RunCommand

getDevice
check device != null
getPlugin
check plugin != null
doCode

Add a helper for this pattern and port existing code

Reviewers: KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka, kdeconnect

Tags: KDE Connect

Differential Revision: https://phabricator.kde.org/D16472