Support React Native 0.56 (#139)

android/build.gradle

@@ -10,13 +10,17 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 android {
-  compileSdkVersion 23
-  buildToolsVersion "23.0.1"
+    compileSdkVersion safeExtGet('compileSdkVersion', 26)
+    buildToolsVersion safeExtGet('buildToolsVersion', '26.0.3')
 
   defaultConfig {
-    minSdkVersion 16
-    targetSdkVersion 23
+    minSdkVersion safeExtGet('minSdkVersion', 16)
+    targetSdkVersion safeExtGet('targetSdkVersion', 26)
     versionCode 1
     versionName "1.0"
   }