[Xamarin.Android.Tools.AndroidSdk] Prefer OpenJDK path on Windows (#31)

Context: https://paper.dropbox.com/doc/OpenJDK-and-You--AH1yWKdVXgno~uXYfmcUAZTwAg-NoECAe2XkBQeoxFfGL6ea

On Windows, as part of the Visual Studio installer, we can install a
Microsoft-maintained OpenJDK.

(This is done principally because Google's Android SDK tooling still
requires JDK 1.8, and Oracle will be making it more difficult to
download and install JDK 1.8 at a future point in time.  We have
deemed it easier to maintain and install our own OpenJDK 1.8 build
so that Google's Android SDK tooling can continue to run.)

Microsoft's OpenJDK install has  priority when determining a default
JDK for developers.

Author: tondat

src/Xamarin.Android.Tools.AndroidSdk/Sdks/AndroidSdkWindows.cs

@@ -103,6 +103,19 @@ protected override IEnumerable<string> GetAllAvailableAndroidSdks ()
 		}
 
 		protected override string GetJavaSdkPath ()
+		{
+			var preferredJdkPath = GetPreferredJdkPath ();
+			if (!string.IsNullOrEmpty (preferredJdkPath))
+				return preferredJdkPath;
+
+			var openJdkPath = GetOpenJdkPath ();
+			if (!string.IsNullOrEmpty (openJdkPath))
+				return openJdkPath;
+
+			return GetOracleJdkPath ();
+		}
+
+		private string GetPreferredJdkPath ()
 		{
 			// check the user specified path
 			var roots = new[] { RegistryEx.CurrentUser, RegistryEx.LocalMachine };
@@ -114,6 +127,26 @@ protected override string GetJavaSdkPath ()
 					return RegistryEx.GetValueString (root, regKey, MDREG_JAVA_SDK, wow);
 			}
 
+			return null;
+		}
+
+		private string GetOpenJdkPath ()
+		{
+			var root = RegistryEx.LocalMachine;
+			var wows = new[] { RegistryEx.Wow64.Key32, RegistryEx.Wow64.Key64 };
+			var subKey = @"SOFTWARE\Microsoft\VisualStudio\Android";
+			var valueName = "JavaHome";
+
+			foreach (var wow in wows) {
+				if (CheckRegistryKeyForExecutable (root, subKey, valueName, wow, "bin", JarSigner))
+					return RegistryEx.GetValueString (root, subKey, valueName, wow);
+			}
+
+			return null;
+		}
+
+		private string GetOracleJdkPath ()
+		{ 
 			string subkey = @"SOFTWARE\JavaSoft\Java Development Kit";
 
 			Logger (TraceLevel.Info, "Looking for Java 6 SDK...");