Fix UnknownPropertyException in getExtOrDefault (#28)

android/build.gradle

 def getExtOrDefault(name, defaultVaue) {
-  return rootProject.ext.get(name) ?: defaultVaue
+  return rootProject.ext.has(name) ? rootProject.ext.get(name) : defaultVaue
 }
 
 apply plugin: 'com.android.library'