[mono] Add Android app sample using mono runtime

[mdh1418]

Summary

This PR looks to port over the Android sample from dotnet/runtime to highlight the concept of building applications on top of the mono runtime.
There are several dependencies such as Android SDK, Android NDK, an Android emulator required to run the sample with dotnet publish.

The sample will depend on a NuGet Package containing the files that build the app and embed the mono runtime into the app, and the code that will build that package can be found here dotnet/runtime#43216

Currently, it is rebased off of #3931 and will be updated after the iOS sample is merged.

[EgorBo]

We probably can get rid of it using Function Pointers for callbacks actually but I'm not sure, last time I checked - callbacks still needed it on Mono.

[EgorBo]

And it's not needed for JIT (which is the only option for Android atm)

[mdh1418]

It works after removing this block as well as the attributes. Does this still qualify as showing how the embedded mono runtime allows for C# and C to call each other with just the function pointers and Dllimport?

[EgorBo]

NOTE: FindClass won't work for Activity in non-UI thread afair.

[mdh1418]

@adegeo Can the check run dotnet publish for these mono-samples rather than dotnet build

samples/.github/workflows/dependencies/Get-MSBuildResults.ps1

Line 138 in 2baf6a9

"dotnet build `"$projectFile`"" | Out-File ".\run.bat"

?

[adegeo]

@mdh1418

@adegeo Andy De George FTE Can the check run dotnet publish for these mono-samples rather than dotnet build

samples/.github/workflows/dependencies/Get-MSBuildResults.ps1

Line 138 in 2baf6a9

"dotnet build `"$projectFile`"" | Out-File ".\run.bat"

?

I just updated the script to allow custom commands. Add the snippets.5000.json file to the same folder as the AndroidSampleApp.csproj file with this content:

{
    "host": "custom",
    "command": "dotnet publish \"$projectFile\""
}

I'm not sure if you need to pull in the latest from master or not.

[mdh1418]

Thanks @adegeo! I think the problems for these mono samples was the github actions runner being windows-latest. I don't think the support to build mono on Windows is there yet, so I opted to add a MacOS lane to verify that the samples build properly via dotnet publish. The related change #3953 would only run for PRs with changes to the mono sample.