Fix issue with Content Page behaviors

[dbergan]

This allows behaviors to be attached to page events via <ContentPage.Behaviors> like Appearing and Disappearing. Without this fix, we get the cryptic following error:

"Additional information: bindable not an instance of AssociatedType"

Original discussion and solution here:
https://forums.xamarin.com/discussion/comment/230200/#Comment_230200

Description of Change

Pull Request Checklist

• Rebased on top of master at time of the pull request.
• Changes adhere to coding standard in the sample.
• Consolidated NuGet packages across all projects in the sample (when changing existing package versions).
• Tested changes on the appropriate platforms (all platforms if changing the library code).

[davidbritch]

Thanks @dbergan