[ResolveSdks] reorder the lookup of JdkJvmPath

[jonathanpeppers]

Steps to Reproduce

Related to #2374

1. Create a new solution in Visual Studio.
2. See a performance regression in the first design-time build. You can also see it in a regular first build, or incremental build if outside the IDE. DTB has the highest performance bar, though.

What should we do?

The code here: https://github.com/xamarin/xamarin-android/blob/e4c15751253ed56cfedfd9d428d6129a5727462a/src/Xamarin.Android.Build.Tasks/Tasks/ResolveSdksTask.cs#L130-L157

1. Move the logic finding JdkJvmPath to a new MSBuild task
2. Don't run the task if $(DesignTimeBuild) is true.
3. Only run the task if $(AndroidGenerateJniMarshalMethods) is true.

/cc @radekdoulik, feel free to take a stab at this if I don't get to it.

VS bug #726927