[Android] Enabling cli config options for android generator

[who]

Now that #805 (thanks @hyeghiazaryan !) has been merged to develop_2.0 (thanks @olensmar !), I've updated the android-java generator to use the new config file functionality.

Here's the output of the android-java config-help :

$ swagger-codegen config-help -l android

CONFIG OPTIONS
    modelPackage
        package for generated models

    apiPackage
        package for generated api classes

    invokerPackage
        root package to use for the generated code

    groupId
        groupId for use in the generated build.gradle and pom.xml

    artifactId
        artifactId for use in the generated build.gradle and pom.xml

    artifactVersion
        artifact version for use in the generated build.gradle and pom.xml

    sourceFolder
        source folder for generated code

    useAndroidMavenGradlePlugin
        A flag to toggle android-maven gradle plugin. Default is true.

I also fixed an issue with gradle variables which was causing build breakages.

[hyeghiazaryan]

+1 to merge this

[olensmar]

looks good to me - waiting for @fehguy