[build] Convert to Xamarin.Legacy.Sdk.

[jpobst]

Context: #247

The first step in multi-targeting AndroidX with net6 is to migrate to Xamarin.Legacy.Sdk, which runs on top of .NET 6 but can build MonoAndroid9.0. We'll commit this first to help make it easier to run our diff logic on the resulting packages to make sure there are no regressions. A future commit will then enable net6.

Changes made:

• CI - Install .NET 6 Preview 5
• CI - Install the Android workload
• Change Cake to use DotNetCore versions
• Update Sdk to Xamarin.Legacy.Sdk

[jpobst]

The only differences seem to be:

• Additional Framework Assembly References
• PDB now contains Compiler Flags

All other files are the same.

I don't think these are issues, I simply raise them for reviewers to verify.

[jonathanpeppers]

The .pdb file looks like an improvement, probably caused by defaults in dotnet/sdk like $(Deterministic).

It doesn't seem like the extra framework references are an issue, maybe they are fine?

I think they are caused by:

https://github.com/xamarin/Xamarin.Legacy.Sdk/blob/f78e04e91651bf6a0021f0c5d483c150506d472e/src/Xamarin.Legacy.Sdk/Sdk/Xamarin.Legacy.Android.targets#L32-L40

So if you just put <Reference Include="Mono.Android"/> those show up.