Don't use args unpacking in call to msg_create_fn.

Summary:
5e1141b837 broke EAS sync because create_eas_message wasn't passing a
namedtuple. I think it's a little clearer to have implementations of
msg_create_fn just take the raw message object and inspect its properties
themselves. This way you can also have the raw message object be any sort of
class, as opposed to a namedtuple specifically.

Test Plan: Ran unit tests; synced test Gmail account.

Reviewers: spang

Reviewed By: spang

Differential Revision: https://review.inboxapp.com/D58