Make this work with RN 56 (#283)

* Make this work with RN 56

* Don't forget to checkin the method too...

android/build.gradle

@@ -1,3 +1,7 @@
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 buildscript {
     repositories {
         jcenter()
@@ -11,12 +15,12 @@ buildscript {
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 25
-    buildToolsVersion "25.0.3"
+    compileSdkVersion safeExtGet('compileSdkVersion', 25)
+    buildToolsVersion safeExtGet('buildToolsVersion', '25.0.3')
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 22)
         versionCode 1
         versionName "1.0"