Fix minor spelling error in build.gradle (#38)

android/build.gradle

-def getExtOrDefault(name, defaultVaue) {
-  return rootProject.ext.has(name) ? rootProject.ext.get(name) : defaultVaue
+def getExtOrDefault(name, defaultValue) {
+  return rootProject.ext.has(name) ? rootProject.ext.get(name) : defaultValue
 }
 
 buildscript {