Add foreign key sample code to example project.

[curioustechizen]

The current example does a great job of demonstrating how to use ContentItem and SimpleContentProvider. It would be great to add an example of using foreign keys in there.

[xxv]

Agreed. There is currently unit test code that does it, but a proper example would be good.

Relatedly, the M2M system is being massively overhauled so that it's easier to use. That'll need an example as well.