Contact Info Widget: pre-fill API key when it has already been set for another Contact Info widget

#### Steps to reproduce the issue

1. Go to Appearance > Widgets
2. Add a Contact Info widget to your site.
3. In that widget, set up the map using a Google Maps API key.
4. Now drag an additional widget to the widget area.

At that point, and since we've already gone through the steps to set up a key, the new widget should already be set to use that key, thus saving us time.

Similarly, it should also be pre-filled when it's already be set thanks to the `jetpack_google_maps_api_key` filter.

Start with the Contact Info widget implementation and trace how the Google Maps API key is stored and how the jetpack_google_maps_api_key filter is applied. Check the behavior when a second widget is added. Done means a new Contact Info widget is pre-filled with an existing key, including one supplied through the filter.

Written by the indexing model from the issue text.