[Xamarin.Android.Build.Tasks] <GenerateJavaStubs/> only for MonoAndroid assemblies

[jonathanpeppers]

Currently the <GenerateJavaStubs/> MSBuild task will load and
iterate over NetStandard or PCL assemblies. It simply looks at all
items in @(_ResolvedUserAssemblies) and writes Java stubs if needed.

We could look at the TargetFrameworkIdentifier and only do this
work for MonoAndroid assemblies.

To make this work:

• The <ResolveAssemblies/> MSBuild task needs to parse the
  TargetFrameworkIdentifier and add it to the item metadata for each
  assembly.
• <GenerateJavaStubs/> should use a subset of
  @(_ResolvedUserAssemblies) where TargetFrameworkIdentifier is
  MonoAndroid.

Results:

Before
    First:       1200 ms  GenerateJavaStubs                          1 calls
    Incremental: 1047 ms  GenerateJavaStubs                          1 calls
After
    First:       1084 ms  GenerateJavaStubs                          1 calls
    Incremental:  868 ms  GenerateJavaStubs                          1 calls

This seems to save ~200ms in the Xamarin.Forms integration project in
this repo. Projects using lots of NetStandard/PCL assemblies should
see a bigger benefit.

[jonathanpeppers]

Binlogs: logs.zip

[jonpryor]

I prefer this original code of using value.StartsWith("MonoAndroid") -- though perhaps it should be extended to value.StartsWith("MonoAndroid,")? -- largely because I'm not sure about what are the requirements and constraints on framework names.

For example, if a "framework name" could contain a comma, the new approach of splitting on all commas may cause a "mismatch". Using StartsWith() against MonoAndroid, would alloy such fears.

[jonathanpeppers]

After using IndexOf + Substring, I think it's still OK to just compare targetFrameworkIdentifier == "MonoAndroid":

//
// Summary:
//     Reports the zero-based index of the first occurrence of the specified string
//     in the current System.String object. A parameter specifies the type of search
//     to use for the specified string.
//
// Returns:
//     The index position of the value parameter if that string is found, or -1 if it
//     is not. If value is System.String.Empty, the return value is 0.
public int IndexOf (String value, StringComparison comparisonType);

It should be the first instance of , and targetFrameworkIdentifier will be everything before it.