notification: Pass policy in the Source contructor

notification: Pass policy in the Source contructor

Commit 932ccac1 changed Source to use a regular constructor
instead of `_init()`.

Unfortunately that results in ordering issues for subclasses
that override `_createPolicy()`, if that method needs to access
any properties that are set in the constructor (as `this` is
only available after chaining up to the parent).

We can fix that by simply setting the policy from the constructor,
instead of relying on some overriden method being called.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3170>