improved SDK values -use SDK Version values from root project

Instead of assuming the `compileSdkVersion `, `targetSdkVersion`, etc, we can make it dynamic by reading it from the root project.


Android Target API Level 26 will be required in August 2018.
https://android-developers.googleblog.com/2017/12/improving-app-security-and-performance.html

Therefore I wrote target values of 26 instead of 23

And the React Native team is already working on this:
facebook/react-native#18095
facebook/react-native#17741

android/build.gradle

@@ -11,15 +11,19 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 android {
-    compileSdkVersion 23
+    compileSdkVersion safeExtGet('compileSdkVersion', 26)
     //noinspection GradleDependency
-    buildToolsVersion "23.0.1"
+    buildToolsVersion safeExtGet('buildToolsVersion', '26.0.3')
 
     defaultConfig {
-        minSdkVersion 16
+        minSdkVersion safeExtGet('minSdkVersion', 16)
         //noinspection OldTargetApi
-        targetSdkVersion 22
+        targetSdkVersion safeExtGet('targetSdkVersion', 26)
         versionCode 1
         versionName "1.0"
     }
@@ -39,5 +43,5 @@ repositories {
 
 dependencies {
     //noinspection GradleDynamicVersion
-    compile 'com.facebook.react:react-native:+'
+    compile "com.facebook.react:react-native:${safeExtGet('reactNativeVersion', '+')}"
 }