Removes two more warnings.

Note that this will ultimately be eliminated when moving to using
OIDCClient.

Tested on Android app deployment to device and UWP on local Windows

The MainTheme set the splash screen image as the window background.
I’ve removed this and ensured that the SplashActivity is the launch
activity.

Also did some code tidy up on the Campaign page classes.

Fixed Checkout process
Changed some UWP icons

This ensures the launch storyboard doesn’t generate an error, and is
line with Xamarin policy for samples.