Don't show a region selector when selecting avatars from a gallery

Summary:
Prompting the user for a region makes sense when loading from a file,
but not from the gallery.

CreateAvatarJob does two things:
- copies from KIO
- goes via a KPixmapRegionSelector and saves to a local file

We know the gallery files are locally mounted, and we don't want the
region selector so we can just skip the job completely.

Test Plan:
Set avatar from file. Got prompt, saved properly
Set avatar from gallery. Skipped prompt, saved properly

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7686