SubscriptionManager.Notification: Split off subclass
ObjectChangedNotification

- In following commits, I will need to introduce special Notification
classes which are not about an object changing, so they won't need
mOldObject / mNewObject. (Specifically, I will add
"BeginSynchroniztationNotification" / "EndSynchronizationNotification"
which will be marker objects to merely mark the begin and end of a
series of synchronization Notifications. Those markers won't contain any
data of their own)
- This commit only starts adapting both classes by showing where their
split point is. The following commits will compile-fix & JavaDoc them.
SubscriptionManager.(ObjectChanged)Notification: Fix constructors

- The constructor of Notification was moved to ObjectChangedNotification
as part of introducing ObjectChangedNotification. This commit moves the
code which belongs to parent class Notification back to it, while
keeping the rest in the child class.
- The visibility of the constructors is trimmed to default visiblity,
which is actually more restricted than "protected".
SubscriptionManager.(ObjectChanged)Notification: Fix
startupDatabaseIntegrityTest()

The startupDatabaseIntegrityTest() of Notification was moved to
ObjectChangedNotification
as part of introducing ObjectChangedNotification. This commit moves the
code which belongs to parent class Notification back to it, while
keeping the rest in the child class.