WebAppInitializer: Move plugin modules loading to after the index one
And load the online upgrader module, if configured so, right after the
plugin module. This change aligns that specific module loading ordering
with the Daemon class, which already implements the right loading order.
This commit basically adjusts what was just previously done for the
WebAppInitializer class, alongside Daemon, in parent commit 020758e.
Bug: Issue 10082