Add a type named "gradleReference" in framework

It's too complex to dependence AAR in plugin, we need build the plugin to a android project which include AndroidManifest.xml. This patch is apply  the build-extras.gradle from plugin to root project. It will be compiled when execute build-extras.gradle in root project. It looks like.

    repositories {
        maven {
            url 'https://download.01.org/crosswalk/releases/crosswalk/android/maven2'
        }
    }
    dependencies {
        compile 'org.xwalk:xwalk_core_library_beta:9.38.208.4'
    }