[Mono.Android] Build latest stable version against netcoreapp3.1

[pjcollins]

Adds netcoreapp3.1 to the set of target frameworks that Mono.Android
will build against in preparation for .NET 5. Additionally, the project build
will effectively be a no-op when targeting netcoreapp3.1 if
$(AndroidFrameworkVersion) is not equal to
$(AndroidLatestStableFrameworkVersion)

[pjcollins]

New build is here, not sure what's up with the duplicate element error above
https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=3569697&view=results

[pjcollins]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jonpryor]

Indentation here is "wonky".

Where should netcoreapp3.1 output go?

Rando thought that I'm trying in PR #4376: place netcoreapp3.1 output into $(topdir)\bin\Debug-netcoreapp3.1.

Better idea -- unimplemented because I don't know the details -- is for files to be included in the .NET 5 support package to follow the appropriate directory structure of the .NET 5 support package. The problem is that I don't know what the directory structure should be...

[pjcollins]

You can find the rough destination package paths here:

• https://github.com/xamarin/xamarin-android/blob/net5-preview/src/Xamarin.Android.App/Xamarin.Android.App.Ref.csproj#L33

• https://github.com/xamarin/xamarin-android/blob/net5-preview/src/Xamarin.Android.App/Xamarin.Android.App.Runtime.csproj#L39

I like the idea of better mirroring this output, though there's a couple of questions/concerns. To do this we'll need to only build the latest stable version of Mono.Android.dll when targeting netstandard2.1, and not every supported version. We also ultimately need Mono.Android.dll in both targeting and runtime packs, so we'll need to account for that in the future as well.

I'm not sure how much of this should be considered for this PR. I took a quick look at attempting to only build the latest Mono.Android.dll question and didn't find a great quick solution to that, but we may want to dedicate time to that aspect now at least so that we don't bloat our build times unnecessarily.

[marek-safar]

Isn't netstandard2.1 too restrictive? You won't be able to use any newer APIs.

[pjcollins]

I've updated the second target to netcoreapp3.1.

[pjcollins]

I fixed the indentation, and changed the output path to use the TargetFramework for now, now that only the latest stable version will be built when targeting netcoreapp3.1 I think we can improve this in the future to better mirror the eventual pack layout once we're compiling against netcoreapp5.0 or net5.0.

[jonpryor]

To do this we'll need to only build the latest stable version of Mono.Android.dll when targeting netstandard2.1, and not every supported version.

The intention is that net5-android will redistribute only API-R/API-30 Mono.Android.dll, and not one for every previous $(TargetFrameworkVersion) value.

[pjcollins]

The latest changes ensure that we only build the $(AndroidLatestStableFrameworkVersion ) version of Mono.Android.dll when targeting netcoreapp3.1.

[pjcollins]

I believe that this reference was only needed to ensure this attribute compiled successfully:

https://github.com/xamarin/xamarin-android/blob/master/src/Mono.Android/Properties/AssemblyInfo.cs.in#L29

This raises another question though, should we still be including these typeforwards, and the System.Drawing sources when targeting netstandard2.1 (or net5 in the future)?

[pjcollins]

I was able to remove the System.Drawing.Common reference when targeting netcoreapp3.1, however I'm still not sure if we should be including the sources/typeforwards when building against this TargetFramework.

[grendello]

There's one problem with this dependency - it's not in standard Debian/Ubuntu/etc repositories, so it will fail on an unprepared Debian machine. I think we should at least handle failure to detect or install it with a descriptive error message pointing to the dotnet website with instructions on how to add the repository.

[pjcollins]

I've removed this, it looks like this .NET Core 3.1 installation issue is happening on other PRs as well, we can look to address this issue in a separate PR.

[jonpryor]

Commit message:

We expect the .NET 5 BCL to closely resemble the .NET Core 3.1 BCL.
Continue preparing for .NET 5 and build `Mono.Android.dll` against
the `netcoreapp3.1` target framework, to ensure that the APIs that we
need actually exist.

Additionally, the `Mono.Android.csproj` build will effectively be a
no-op when targeting `netcoreapp3.1` if `$(AndroidFrameworkVersion)`
is not equal to `$(AndroidLatestStableFrameworkVersion)`, as we
currently plan on only shipping *one* `Mono.Android.dll` assembly as
part of .NET 5, so there is no need to build `Mono.Android.dll`
against other `$(TargetFrameworkVersion)` values.

When building for `netcoreapp3.1`, `Mono.Android.dll` uses the
`Java.Interop.dll` which is built for .NET Standard, instead of the
one built against the `MonoAndroid` framework.  We may want to
consider updating `Java.Interop.csproj` to multitarget against
`netcoreapp3.1` as well.