Mass update to Xamarin.Forms 4.5

[davidbritch]

Description of Change

A mass update from Xamarin.Forms 4.4 to 4.5, including build fixes.

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).