Support D8 for dex file compilation

[cobey]

D8 is now suggested dex compiler for Android development, with enhanced speed and reduced file sizes. We should aim to support D8 as the dex compiler.

VS bug #660386

[atsushieno]

There is already a solution for d8. https://github.com/atsushieno/xamarin-android-d8-build

[cobey]

@jonpryor for context

We want this to be a default experience within Xamarin.Android. While the solution presented is a fine interim solution, we want the improvements that d8 offers as default for 15.8

[atsushieno]

I have added D8 support within xamarin-android itself so that users don't have to install my nuget package independently. Instead, <AndroidEnableD8>true</AndroidEnableD8> is required in the app .csproj.

I don't think enabling D8 by default at this stage is appropriate. What is doable instead is to add the property above in the default Android app project templates.

[atsushieno]

(I mean, added to #1489)

[JonDouglas]

@atsushieno When can we bump our templates to include the property by default? #1489 is still outstanding, so I would guess 15.9?

[atsushieno]

Can we please fix any blocking issue first so that I can continue the work?

[JonDouglas]

Closing this issue in favor of #2040