Fix support for RN 0.56+ by modernizing the build.gradle.

Author: mcrowe

android/build.gradle

@@ -1,22 +1,26 @@
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 buildscript {
     repositories {
         jcenter()
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:1.1.3'
+        classpath 'com.android.tools.build:gradle:2.2.3'
     }
 }
 
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion safeExtGet('compileSdkVersion', 26)
+    buildToolsVersion safeExtGet('buildToolsVersion', '26.0.3')
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 26)
         versionCode 1
         versionName "1.0"
     }
@@ -31,4 +35,4 @@ repositories {
 
 dependencies {
     compile 'com.facebook.react:react-native:0.12.+'
-}
+}