Add a section for plugin extensions

The build.gradle will apply gradle srcipte from plugin extension
When install the plugin with "gradleReference" framework.
The gradle can set ext.multiarch=true to support multiple APKs by
default, so add this section in here.

bin/templates/project/build.gradle

@@ -40,6 +40,9 @@ task wrapper(type: Wrapper) {
     gradleVersion = '1.12'
 }
 
+// PLUGIN GRADLE EXTENSIONS START
+// PLUGIN GRADLE EXTENSIONS END
+
 ext.multiarch=false
 
 android {