Rename IOS to APNs in function responsible for notifications encryption

Summary:
This differential renames functions responsible for notification encryption to use "APNs" instead of "IOS" to emphasize that notifications encryption will
cover all Apple devices.

Test Plan: Ensure that flow and eslint do not complain.

Reviewers: michal

Reviewed By: michal

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10056