[build] stop installing Xamarin.Android

[jonathanpeppers]

Context: https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=10326490&view=logs&j=293e774d-1b3c-56a2-2db1-26d172f04ddd&t=b180d07a-099b-5dc9-8d17-5c88abaf95ba&l=11

The attempt to install Xamarin.Android is failing on macOS anyway, as it relies on a VS Mac updater service:

Querying https://software.xamarin.com/Service/Updates?v=2&pv964ebddd-1ffe-47e7-8128-5ce17ffffb05=0&pv4569c276-1397-4adb-9485-82a7696df22e=0&pvd1ec039f-f3db-468b-a508-896d7c382999=0&pv0ab364ff-c0e9-43a8-8747-3afb02dc7731=0&level=Stable
Retry attempt 1: System.Net.Http.HttpRequestException: Response status code does not indicate success: 404 (Not Found).
at System.Net.Http.HttpResponseMessage.EnsureSuccessStatusCode()
at Boots.Core.HttpClientWithPolicy.DoGetXmlDocumentAsync(Uri uri, CancellationToken token) in D:\a\1\s\Boots.Core\HttpClientWithPolicy.cs:line 59
at Polly.Timeout.AsyncTimeoutEngine.ImplementationAsync[TResult](Func`3 action, Context context, CancellationToken cancellationToken, Func`2 timeoutProvider, TimeoutStrategy timeoutStrategy, Func`5 onTimeoutAsync, Boolean continueOnCapturedContext)
at Polly.Timeout.AsyncTimeoutEngine.ImplementationAsync[TResult](Func`3 action, Context context, CancellationToken cancellationToken, Func`2 timeoutProvider, TimeoutStrategy timeoutStrategy, Func`5 onTimeoutAsync, Boolean continueOnCapturedContext)
at Polly.AsyncPolicy.ExecuteAsync[TResult](Func`3 action, Context context, CancellationToken cancellationToken, Boolean continueOnCapturedContext)
at Polly.Wrap.AsyncPolicyWrapEngine.<>c__DisplayClass3_0`1.<<ImplementationAsync>b__0>d.MoveNext()
--- End of stack trace from previous location ---
at Polly.Retry.AsyncRetryEngine.ImplementationAsync[TResult](Func`3 action, Context context, CancellationToken cancellationToken, ExceptionPredicates shouldRetryExceptionPredicates, ResultPredicates`1 shouldRetryResultPredicates, Func`5 onRetryAsync, Int32 permittedRetryCount, IEnumerable`1 sleepDurationsEnumerable, Func`4 sleepDurationProvider, Boolean continueOnCapturedContext)

This does still work on Windows, taking about 5 minutes:

Inferring .vsix from URL.
Downloading https://download.visualstudio.microsoft.com/download/pr/a3846965-8f4c-42fa-b728-b6ea5f0a2a16/90d568134f0f5b472e5c085fc6573c76bd40231892a89eb7551940dcc055eb8a/Xamarin.Android.Sdk-13.2.2.0.vsix
Writing to C:\Users\cloudtest\AppData\Local\Temp\rpcpoivh.dej.vsix
Using path from vswhere: C:\Program Files\Microsoft Visual Studio\2022\Enterprise
...
Deleting C:\Users\cloudtest\AppData\Local\Temp\rpcpoivh.dej.vsix
Finishing: install Xamarin.Android stable

We probably don't even need to isntall it anymore. So, let's just remove this step.

[pjcollins]

I was trying to find any tests that may still rely on classic and think the last few were removed in 437f774.

The BuildBasicApplicationWithNuGetPackageConflicts test still references old monoandroid* target framework identifiers, but this seems inconsequential and can probably be cleaned up separately.