Move gaia component extension initialization closer to where it is really needed
SigninScreenHandler is used for showing OOBE and account picker screen so it is too early
to init the extension. GaiaScreenHandler::LoadGaia is called when GAIA screen is about
to show so it is right time to load the extension. On login screen after this change
the extension loads after account picker is ready.
BUG=348548
TEST=manual + existing tests
Review URL: https://codereview.chromium.org/
723523002
Cr-Commit-Position: refs/heads/master@{#303833}