Bump to xamarin/xamarin-android-tools/master@3974fc38

Changes: dotnet/android-tools@f5fcb9f...3974fc3

  * dotnet/android-tools@3974fc3: [Xamarin.Android.Tools.AndroidSdk] JdkInfo + JDK11 + Windows (dotnet#88)
  * dotnet/android-tools@5552b07: [Xamarin.Android.Tools.AndroidSdk] Improve utility of JDK warnings (dotnet#87)
  * dotnet/android-tools@13cc497: [Xamarin.Android.Tools.AndroidSdk] Prefer JAVA_HOME (dotnet#86)
  * dotnet/android-tools@967c278: Delete NuGet.Config
  * dotnet/android-tools@2d3690e: [Xamarin.Android.Tools.AndroidSdk] Nullable Reference Type support (dotnet#84)
  * dotnet/android-tools@2020b20: [Xamarin.Android.Tools.AndroidSdk] Preview Build-Tools are Last (dotnet#85)

Author: jonpryor