Fix some build issues

[mmarinchenko]

This PR is part 1 of #437. It fixes some build issues related to main source directory.

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build.cake changes

1. Addins and tools versions are bumped up. Message about older Cake.Core is no longer appears in the build log.
2. No need to have java executable in the PATH directly. JAVA_HOME/bin/java may be used to generate javadocs now.
3. No need to have nuget executable in the PATH nor install NuGet.Versioning addin. MSBuild is always used to restore projects now.
4. MSBuild is always uses specified MAX_CPU_COUNT now.
5. Unused code which caused CS0162 and CS0219 is commented out.
6. Some tabs vs. spaces mess is fixed.

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Related changes

1. There was "unused-almost-empty" project Firebase-Iid.csproj. I deleted it.
2. GooglePlayServicesTargets.cshtml had unnecessary duplicated tags.
3. <NoWarn> tags were filled incorrectly in GooglePlayServicesProject.cshtml (I also added BG8A01 in addition to BG8A00 and BG8A04). Some of them didn't work earlier - so I added TODOs to review them as they may suppress potentially important warnings now.

[moljac]

Some of them didn't work earlier - so I added TODOs to review them as they may suppress potentially important warnings now.

It still does not work on Mac, but work on Windows (WarningsAsErrors) - found out this week after build started mysteriously failing on Windows CI. Not sure what was changed.

[mmarinchenko]

@moljac

It still does not work on Mac, but work on Windows (WarningsAsErrors) - found out this week after build started mysteriously failing on Windows CI. Not sure what was changed.

I don't have access to your Azure Pipelines but it seems macOS and Windows environments may have different Java.Interop versions. I started having problems with Kotlin interfaces nested in private/internal classes after I upgraded to VS 16.9 (i.e. Xamarin.Android 11.2) on Windows.

I'm not sure but some of these changes may be related: https://github.com/xamarin/java.interop/commits/main/src/Xamarin.Android.Tools.Bytecode/Kotlin/KotlinFixups.cs

[moljac]

can you revert to latest 29.x.y build tools instead of 30.x.y, please?

The builds are failing

       "/Users/runner/work/1/s/generated/GooglePlayServices.sln" (Build target) (1:2) ->
       "/Users/runner/work/1/s/generated/com.google.mlkit.vision-internal-vkp/com.google.mlkit.vision-internal-vkp.csproj" (default target) (140:10) ->
         /Library/Frameworks/Mono.framework/External/xbuild/Xamarin/Android/Xamarin.Android.Legacy.targets(176,5): error XA0032: Java SDK 11.0 or above is required when using Android SDK Build-Tools 30.0.2. [/Users/runner/work/1/s/generated/com.google.mlkit.vision-internal-vkp/com.google.mlkit.vision-internal-vkp.csproj]

[mmarinchenko]

Yes, but that means you're using Xamarin.Android 11.1 or older. I guess my assumption about different Java.Interop versions on macOS and Windows is correct)

xamarin/xamarin-android#5234

[mmarinchenko]

@moljac

I've created xamarin/java.interop#826.

[jpobst]

This PR is likely outdated and no longer relevant.