Refactor the DummyAvatarService into two services

Review Request #8526 — Created Nov. 7, 2016 and submitted — Latest diff uploaded

Information

Djblets
release-0.10.x

Reviewers

The DummyAvatarService no longer does magic to set whether or not it
should provide high DPI avatars. Instead, we now have the
DummyHighDPIAvatarService to provide that (and the
DummyAvatarService will not provide high DPI avatars).

Ran unit tests.