Fix update logic for multi backend

This change fixes the logic to update services in
AbstractMasterSystemService when a derviced class is configured to
support multiple backends. Before this change, after an update
all PerUserService instances would point to the same backend.

Test: Built & deployed locally

Change-Id: Icfb5860bd0baa588b914375ddfae88741b8fcb9c
2 files changed