FastDev should alter $(RuntimeIdentifiers) for connected device

[jonathanpeppers]

Right now in .NET 6 an empty app builds for two architectures:

dotnet new android
dotnet build

This produces a Debug .apk for arm64-v8 and x86 that would run on most devices/emulators.

If you have an x86 device selected in Visual Studio, we should have some behavior that will:

1. Clear $(RuntimeIdentifiers) plural
2. Set a single $(RuntimeIdentifier) matching the connected device

We already have similar behavior for "legacy" Xamarin.Android, but not .NET 6.

This would speed up build times for Debug builds in .NET 6.

[dellis1972]

@jonathanpeppers how early on in the build process will GetPrimaryCpuAbi need to run in .net 6 for us to be able change RuntimeIdentifiers ?

[jonathanpeppers]

I think as long as $(RuntimeIdentifier) and $(RuntimeIdentifiers) are updated before this target runs:

https://github.com/xamarin/xamarin-android/blob/549f2ced9e9c88cb8d7c1599d8e974976e90eb9d/src/Xamarin.Android.Build.Tasks/Microsoft.Android.Sdk/targets/Microsoft.Android.Sdk.AssemblyResolution.targets#L44-L48

We probably want to clear $(RuntimeIdentifiers) plural and set a single $(RuntimeIdentifier).