A few tweaks to PurpleAccountManager

Add a weak pointer on the default_manager instance to clear itself.
Make purple_account_manager_get_default_as_model safe if default_manager is NULL.
Fix the method names for the GListModel implementation.
Mark the type as final.

Testing Done:
Ran the tests and docs.

Reviewed at https://reviews.imfreedom.org/r/2083/