Make service account file & credentials optional arguments, clean up BaseAPI scope

[J-Priebe]

See related issue

Tighten up the interface by accepting Credentials directly without the need to pass in falsey values for project_id and service_account_file.

Additionally some minor cleanup to BaseAPI:

• Remove variables that were unnecessarily part of the outer class scope
• lowercase self.fcm_end_point to distinguish instance variable from the FCM_END_POINT_BASE constant