Using SDK Version variables from root project

Instead of assuming the `compileSdkVersion`, `targetSdkVersion`, etc, read it from the root project.
Default `compileSdkVersion` and `targetSdkVersion` to the latest versions.

Android Target API Level 26 will be required in August 2018.
https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html
And the React Native team is already working on this:
facebook/react-native#17741
facebook/react-native#18095

android/build.gradle

@@ -1,20 +1,32 @@
 apply plugin: 'com.android.library'
 
+def _ext = rootProject.ext
+
+def _reactNativeVersion = _ext.has('reactNative') ? _ext.reactNative : '+'
+def _compileSdkVersion = _ext.has('compileSdkVersion') ? _ext.compileSdkVersion : 27
+def _buildToolsVersion = _ext.has('buildToolsVersion') ? _ext.buildToolsVersion : '27.0.3'
+def _minSdkVersion = _ext.has('minSdkVersion') ? _ext.minSdkVersion : 16
+def _targetSdkVersion = _ext.has('targetSdkVersion') ? _ext.targetSdkVersion : 27
+
 android {
-    compileSdkVersion 25
-    buildToolsVersion "25.0.2"
+    compileSdkVersion _compileSdkVersion
+    buildToolsVersion _buildToolsVersion
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion _minSdkVersion
+        targetSdkVersion _targetSdkVersion
         versionCode 1
         versionName "1.0"
         ndk {
             abiFilters "armeabi-v7a", "x86"
         }
     }
+    lintOptions {
+        abortOnError false
+    }
 }
 
 dependencies {
-    compile 'com.facebook.react:react-native:+'
-}
+    //noinspection GradleDynamicVersion
+    compile "com.facebook.react:react-native:${_reactNativeVersion}"
+}