Restructure the default dependency.

dependencies/extension-api/build.gradle

@@ -0,0 +1,16 @@
+buildscript {
+	repositories {
+		mavenCentral()
+	}
+	
+	dependencies {
+		classpath 'com.android.tools.build:gradle:2.1.0'
+	}
+}
+
+apply plugin: 'android-library'
+
+android {
+	compileSdkVersion Integer.parseInt(project.ANDROID_BUILD_SDK_VERSION)
+	buildToolsVersion project.ANDROID_BUILD_TOOLS_VERSION
+}

dependencies/extension-api/build.xml

@@ -1,13 +0,0 @@
-<?xml version="1.0" encoding="UTF-8"?>
-<project name="OpenFLExtension" default="help">
-	
-	<property environment="env"/>
-	<property name="sdk.dir" value="${env.ANDROID_SDK}"/>
-	
-	<property file="local.properties"/>
-	<property file="ant.properties"/>
-	<property file="project.properties"/>
-	
-	<import file="${sdk.dir}/tools/ant/build.xml"/>
-	
-</project>

dependencies/extension-api/project.properties

@@ -1,13 +0,0 @@
-# This file is automatically generated by Android Tools.
-# Do not modify this file -- YOUR CHANGES WILL BE ERASED!
-# 
-# This file must be checked in Version Control Systems.
-# 
-# To customize properties used by the Ant build system use,
-# "build.properties", and override values to adapt the script to your
-# project structure.
-
-android.library=true
-
-# Project target.
-target=android-::ANDROID_TARGET_SDK_VERSION::