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

@@ -13,12 +13,21 @@ buildscript {
     }
 }
 
+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 23
-    buildToolsVersion "23.0.1"
+    compileSdkVersion _compileSdkVersion
+    buildToolsVersion _buildToolsVersion
+
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 23
+        minSdkVersion _minSdkVersion
+        targetSdkVersion _targetSdkVersion
         versionCode 1
         versionName "1.0"
     }
@@ -33,6 +42,7 @@ android {
 }
 
 dependencies {
-    compile 'com.facebook.react:react-native:+'
+    //noinspection GradleDynamicVersion
+    compile "com.facebook.react:react-native:${_reactNativeVersion}"
     //{RNFetchBlob_PRE_0.28_DEPDENDENCY}
 }

android/gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,6 @@
-#Wed May 18 12:33:41 CST 2016
+#Tue Apr 24 14:22:15 BRT 2018
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-2.10-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-2.14.1-all.zip