Change buildToolsVersion to config by rootProject

build.gradle

@@ -14,7 +14,7 @@ apply plugin: 'com.android.library'
 
 android {
     compileSdkVersion 25
-    buildToolsVersion '26.0.2'
+    buildToolsVersion rootProject.ext.buildToolsVersion
 
     compileOptions {
         sourceCompatibility JavaVersion.VERSION_1_8