Implement function to encrypt android notification

Summary:
This differential implements function that can encrypt Android notification. Additionally it introduces type for Android notification. The type is very flexible (so is Android notification payload) but its main purpose is to
protect against uninformed developer that might not know that olm doesn't accept anything else than strings. Additionally it protects against attempts to encrypt notification id.

Test Plan: Use this funcion in prepareAndroidNotification. Log the result, examine its content.

Reviewers: tomek, atul, bartek, ashoat

Reviewed By: tomek, ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D7872