Fix Samples to Unified Namespace Update, and other sample clean-up

[michael-hawker]

We accidentally broke the samples with #286 and #282 as they're in a separate solution not referenced in the main solution.

This PR intends to start fixing that, first:

• Building samples solution in GitHub CI for validation
• Remove old duplicate mobile page copies from sample
• Fix samples building by updating to new pattern for 3.0 with unified namespace
• Update sample dependency
• Remove pfx props
• Use file-scoped namespaces (shouldn't conflict with Switch all files to file-scoped namespaces #290 as that just does src and we just do the samples)
• Clean-up usings

Just wanted to get the CI to validate this first before digging in so we can validate the final result.

[michael-hawker]

As expected:

"D:\a\XamlBehaviors\XamlBehaviors\samples\CS\XAMLBehaviorsSample.sln" (default target) (1) ->
(Build target) ->
D:\a\XamlBehaviors\XamlBehaviors\samples\CS\XAMLBehaviorsSample.sln.metaproj : error MSB3202: The project file "D:\a\XamlBehaviors\XamlBehaviors\src\BehaviorsSDKManaged\Microsoft.Xaml.Interactions\Microsoft.Xaml.Interactions.csproj" was not found. [D:\a\XamlBehaviors\XamlBehaviors\samples\CS\XAMLBehaviorsSample.sln]

Will pick this up again tomorrow afternoon, hopefully.

[Sergio0694]

Looks good, thank you!!