Is there way to get the APPIAN_HOME value thru java APIs in plugin code. I went

What exactly do you need to get from Appian folders? Maybe there are other ways, such as getting from classpath.

Also you could try to get "current" directory, and from there, you can come up with relative path to the folder inside Appian directory structure:

Path currentRelativePath = Paths.get("");
String s = currentRelativePath.toAbsolutePath().toString();