Add encryptedNotifUtilsAPI on client platforms with mock encryption.

Summary: This differential introduces encryptedNotifUtilsAPI on the client platforms to smoothly use function from lib/push. Encryption implementation is mocked and will be replaced once we tackle notif session initialization.

Test Plan: Test plan for D12464 but without adding encrytpedNotifUtilsAPI implementation to the patch.

Reviewers: kamil, tomek

Reviewed By: tomek

Subscribers: bartek, ashoat

Differential Revision: https://phab.comm.dev/D12572