Preferences helper will call `_initialize` twice at construction

[itziakos]

The current implementation will call _initialize during the HasTraits contructor and at the same time the static change handler on the preferences trait would have already triggered the same call when the parent constructor is called.

While this behavior is normally harmless it is problematic and unnecessary.

https://github.com/enthought/apptools/blob/94c4963f8aa9ea5d52f1d5c8020df54db1c96524/apptools/preferences/preferences_helper.py#L39