[Windows] check %JAVA_HOME% for locating Jdk (#74)

* [Windows] check %JAVA_HOME% for locating Jdk

Fixes: http://feedback.devdiv.io/606218

Something has happened with Xamarin.Android builds running on Azure
DevOps recently, builds are now failing with:

    ResolveSdks
        ...
    Errors
        C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Xamarin\Android\Xamarin.Android.Common.targets(721,2): error XA5300: The Java SDK Directory could not be found. Please set via /p:JavaSdkDirectory. [d:\a\1\s\Xamarin.Android.Lite\Xamarin.Android.Lite.csproj]

Azure DevOps has posted a "workaround" document:

https://github.com/microsoft/azure-pipelines-image-generation/blob/1371aa8548aa135c3be783bd854681118ca10f78/images/win/Vs2019-Server2019-Readme.md#known-issues

It suggests adding this property to MSBuild calls:

    /p:JavaSdkDirectory="$(JAVA_HOME_8_X64)"

Reviewing a user's build log from ReactiveUI:

https://dev.azure.com/dotnet/ReactiveUI/_build/results?buildId=18505

    /p:JavaSdkDirectory=C:/Program%20Files/Java/zulu-8-azure-jdk_8.38.0.13-8.0.212-win_x64

So I tried this with my own repo that has Azure DevOps configured,
sure enough a build from June 18, 2019 worked. A rebuild on July 1,
2019 failed with the reported error.

* Worked: https://jopepper.visualstudio.com/Jon%20Peppers%20OSS/_build/results?buildId=208
* Failed: https://jopepper.visualstudio.com/Jon%20Peppers%20OSS/_build/results?buildId=209

I can read from the build logs that both `JAVA_HOME` and
`JAVA_HOME_8_X64` are set to:

    C:/Program Files/Java/zulu-8-azure-jdk_8.38.0.13-8.0.212-win_x64

I think this means two things:

* Microsoft Open JDK is no longer installed on the `Hosted Windows
  2019 with VS2019` pool.
* Instead, *yet another JDK* called the Azul Zulu JDK is present...

https://docs.microsoft.com/en-us/java/azure/jdk/java-jdk-install?view=azure-java-stable

I think we should add a "last resort" lookup for a valid JDK within
`JAVA_HOME` on Windows. It does not currently do that.

Reviewing the code, I am somewhat confused by `JdkInfo`. Its logic is
not run on Windows, and instead we have Windows-specific JDK lookups
in `AndroidSdkWindows`. To make things work properly we can just add a
last chance lookup for `JAVA_HOME` in `AndroidSdkWindows`.

I added a new test for verifying things locally, however...

I can't actually run this test on CI, since our current CI machines have the Microsoft Open JDK installed.

Author: jonathanpeppers

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

@@ -135,7 +135,19 @@ IEnumerable<JdkInfo> ToJdkInfos (IEnumerable<string> paths, string locator)
 			return ToJdkInfos (GetPreferredJdkPaths (), "Preferred Registry")
 				.Concat (ToJdkInfos (GetOpenJdkPaths (), "OpenJDK"))
 				.Concat (ToJdkInfos (GetKnownOpenJdkPaths (), "Well-known OpenJDK paths"))
-				.Concat (ToJdkInfos (GetOracleJdkPaths (), "Oracle JDK"));
+				.Concat (ToJdkInfos (GetOracleJdkPaths (), "Oracle JDK"))
+				.Concat (ToJdkInfos (GetEnvironmentJdkPaths (), "Environment Variables"));
+		}
+
+		private static IEnumerable<string> GetEnvironmentJdkPaths ()
+		{
+			var environment = new [] { "JAVA_HOME" };
+			foreach (var key in environment) {
+				var value = Environment.GetEnvironmentVariable (key);
+				if (!string.IsNullOrEmpty (value)) {
+					yield return value;
+				}
+			}
 		}
 
 		private static IEnumerable<string> GetPreferredJdkPaths ()

src/Xamarin.Android.Tools.AndroidSdk/Tests/AndroidSdkInfoTests.cs

@@ -135,6 +135,35 @@ public void Constructor_SetValuesFromPath ()
 			}
 		}
 
+		[Test]
+		[Ignore ("This test will only work locally if you rename/remove your Open JDK directory.")]
+		public void JdkDirectory_JavaHome ()
+		{
+			CreateSdks (out string root, out string jdk, out string ndk, out string sdk);
+			JdkInfoTests.CreateFauxJdk (jdk, releaseVersion: "1.8.999", releaseBuildNumber: "9", javaVersion: "1.8.999-9");
+
+			var logs = new StringWriter ();
+			Action<TraceLevel, string> logger = (level, message) => {
+				logs.WriteLine ($"[{level}] {message}");
+			};
+
+			string java_home = null;
+			try {
+				// We only set via JAVA_HOME
+				java_home = Environment.GetEnvironmentVariable ("JAVA_HOME", EnvironmentVariableTarget.Process);
+				Environment.SetEnvironmentVariable ("JAVA_HOME", jdk);
+				var info = new AndroidSdkInfo (logger, androidSdkPath: sdk, androidNdkPath: ndk, javaSdkPath: "");
+
+				Assert.AreEqual (ndk, info.AndroidNdkPath, "AndroidNdkPath not preserved!");
+				Assert.AreEqual (sdk, info.AndroidSdkPath, "AndroidSdkPath not preserved!");
+				Assert.AreEqual (jdk, info.JavaSdkPath, "JavaSdkPath not preserved!");
+			} finally {
+				if (java_home != null)
+					Environment.SetEnvironmentVariable ("JAVA_HOME", java_home, EnvironmentVariableTarget.Process);
+				Directory.Delete (root, recursive: true);
+			}
+		}
+
 		static  bool    IsWindows   => OS.IsWindows;
 
 		static void CreateSdks (out string root, out string jdk, out string ndk, out string sdk)