[Windows] check %JAVA_HOME% for locating Jdk

[jonathanpeppers]

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 also cleaned up the tests a bit: CreateFauxJavaSdkDirectory was
not setting up jar.exe or jvm.dll.