Automatically grab Java home from the JRE location in System.properties ...

...if set.

Android Studio sets this in the JDK Location box in its Project Settings, and does not use JAVA_HOME. Android Studio then passes this location to Gradle. This patch enables retrolambda to read this setting, freeing the user from having to set a JAVA_HOME environment variable, which is the current configuration suggestion.

Tested this with my Android Studio and it works perfectly! With my patched version of retrolambda, I can use lambdas in my Android project **WITHOUT** JAVA_HOME or JAVA8_HOME defined and **WITHOUT** the following configuration block:

    retrolambda {
        jdk System.getenv("JAVA8_HOME")
        oldJdk System.getenv("JAVA7_HOME")
        javaVersion JavaVersion.VERSION_1_7
    }
Don't have to move into a function, but you should make it private since we don't want to expose this to plugin users.