messageTray: Split out policy creation

messageTray: Split out policy creation

The _createPolicy() method of a subclass usually depends on some
constructor parameters that need to be set before chaining up to
the parent. This works fine with Lang.Class, but will break with
ES6 classes, as "this" is only initialized after chaining up.

Prepare for this by not creating the policy in the constructor,
but when it is first accessed.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/350