[Architecture] Do sync framework integration in a separate class

[rfc2822]

Create a new class for SyncFrameworkIntegration, which is used for all sync framework integration (for contacts, calendars etc.) Goal is that other classes never call ContentResolver.setIsSyncable() or something like that themselves. Everything sync-framework related must be handled by this class.

The sync framework must only be used to get notified about changed content provider entries. All other users should be disabled by the new class (like current behaviour).

It should also be kept in mind that if we would ever like to change the notification of changed content provider entries to JobScheduler / WorkManager (https://github.com/bitfireAT/davx5/issues/247), it should be possible by adapting mainly this class.