Fix wrong build tools version in RN 0.56

Author: marekpiechut

android/build.gradle

@@ -1,12 +1,12 @@
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 27
-    buildToolsVersion "27.0.3"
+    compileSdkVersion rootProject.ext.compileSdkVersion
+    buildToolsVersion rootProject.ext.buildToolsVersion
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 27
+        minSdkVersion rootProject.ext.minSdkVersion
+        targetSdkVersion rootProject.ext.targetSdkVersion
         versionCode 1
         versionName "1.0"
         ndk {