StartParameter.setGradleUserHomeDir does not take GRADLE_USER_HOME into account

The method claims to use set to a default value if null is passed, but it's logic is just use DEFAULT_GRADLE_USER_HOME, while the StartParameter constructor has more advanced logic to set a default value (using the GRADLE_USER_HOME environment variable and the gradle.user.home property). The two are inconsistent. The code in the constructor should be moved out to a method to be used by the constructor and the setGradleUserHomeDir method.