housekeeping: Add Analyzers to the Sample App

[worldbeater]

What kind of change does this PR introduce?

This PR adds analyzers to our LoginApp which is our sample app. Also, this PR fixes the warnings produced by the analyzers. Additionally, we are now referencing ReactiveUI.Validation as <PackageReference Include="ReactiveUI.Validation" Version="1.8.*" />, this should allow folks to run the sample app without the need to install dozens of frameworks and libraries, so this should make the samples more accessible. Finally, this adds an IUserDialogs implementation to our new LoginApp.Android project introduced in #138

What is the current behavior?

Currently, there are no analyzers in our sample LoginApp.

What is the new behavior?

Now, the analyzers are added.

What might this PR break?

Nothing.

[codecov]

Codecov Report

Merging #143 into main will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #143   +/-   ##
=======================================
  Coverage   61.57%   61.57%           
=======================================
  Files          16       16           
  Lines         877      877           
=======================================
  Hits          540      540           
  Misses        337      337           

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