Have more contrast in circle-style user avatars

Summary:
Our circular avatar currently don't have the best contrast between their background and main element, which is especially evident when using light themes.

This patch raises the opacity of the background circle, making it more prominent.

Test Plan:
Before & After (Breeze):

Before & After (Breeze Dark)

Reviewers: VDG, ngraham, ndavis

Reviewed By: VDG, ngraham, ndavis

Subscribers: ngraham, plasma-devel

Tags: Plasma

Maniphest Tasks: T9910

Differential Revision: https://phabricator.kde.org/D21926