When updating another user's avatar don't save icon into our home

Summary:
We want to save the face using AccountsService, but for backwards
compatibility we also save the icon into ~/.face for old
apps/DisplayManagers that still expect that.

This works when setting a face as the current user, but doesn't make
sense when setting the icon of another user.

We can't fix it to write into another user's home, so instead of trying
just skip that step and only update AccountsService.

BUG: 365662
BUG: 360005
BUG: 371663

Test Plan:
Changed my user's avatar. AccountsService updated ~/.face updated

Changed another user's avatar from my account. Accounts service updated, ~/.face was
unchanged

Reviewers: #plasma, mart

Reviewed By: mart

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4036