Load user information once on startup

Summary:
Apparently the seccomp filter can break KUser in certain setups, so load the
info before the sandbox is active.

Test Plan:
Provided a test package to the reporter on the downstream bug
(https://bugzilla.opensuse.org/show_bug.cgi?id=1160995), who reported
success.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26722