[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)
 returnToJdkInfos(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"));
+}
+
+privatestaticIEnumerable<string>GetEnvironmentJdkPaths()
+{
+varenvironment=new[]{"JAVA_HOME"};
+foreach(varkeyinenvironment){
+varvalue=Environment.GetEnvironmentVariable(key);
+if(!string.IsNullOrEmpty(value)){
+yieldreturnvalue;
+}
+}
 }
 
 privatestaticIEnumerable<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.")]
+publicvoidJdkDirectory_JavaHome()
+{
+CreateSdks(outstringroot,outstringjdk,outstringndk,outstringsdk);
+JdkInfoTests.CreateFauxJdk(jdk,releaseVersion:"1.8.999",releaseBuildNumber:"9",javaVersion:"1.8.999-9");
+
+varlogs=newStringWriter();
+Action<TraceLevel,string>logger=(level,message)=>{
+logs.WriteLine($"[{level}] {message}");
+};
+
+stringjava_home=null;
+try{
+// We only set via JAVA_HOME
+java_home=Environment.GetEnvironmentVariable("JAVA_HOME",EnvironmentVariableTarget.Process);
+Environment.SetEnvironmentVariable("JAVA_HOME",jdk);
+varinfo=newAndroidSdkInfo(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);
+}
+}
+
 staticboolIsWindows=>OS.IsWindows;
 
 staticvoidCreateSdks(outstringroot,outstringjdk,outstringndk,outstringsdk)