[Push] Change mechanism to fetch/update Push settings

[cketti]

• Add an abstraction (PushManager?) that will call PushRegistrationManager.update() whenever Push settings are changed. PushDistributorManager and PushRegistrationManager will become implementation details and should not be used directly.
• Expose Push settings as a Flow to which the app settings and Push settings screens can subscribe.

[sunkup]

PushRegistrationManager is used from RefreshCollectionsWorker and other locations, which, if everything should go through PushManager, should also use the PushManager then.