Dolphin Service Menu installer cleanup

Summary:
This patch contains some smaller formatting/coding style changes
to avoid unrelated changes in other patches.

- Use {} instead of QStringList{} consistently
- Use parehthesese in `UncompressCommand({})` consistently
- Use `QStringList name = {}` instead of `auto name = QStringList{}`
- Add one qAsConst, remove qAsConst if value is already const

Test Plan: Compile, install and run tests

Reviewers: #dolphin, ngraham, nicolasfella

Subscribers: kfm-devel

Tags: #dolphin

Differential Revision: https://phabricator.kde.org/D28801