This now uses e_source_get_oauth2_access_token_sync() instead of talking
directly to GNOME Online Accounts and therefore no longer has to live in
the "online-accounts" module.

The "online-accounts" module will be removed in Evolution 3.9.1, when we
can require GNOME Online Accounts 3.8.  Evolution 3.8 will still support
older versions of GNOME Online Accounts, which use OAuth 1.0a tokens for
Google authentication.