Using SDK Version variables from root project (#714)

* 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

* Renaming variables for consistency

* Rename variables and use *Version for all settings

android/build.gradle

@@ -10,13 +10,21 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
+def ext = rootProject.ext
+
+def projectReactNativeVersion = ext.has('reactNativeVersion') ? ext.reactNativeVersion : '+'
+def projectCompileSdkVersion = ext.has('compileSdkVersion') ? ext.compileSdkVersion : 27
+def projectBuildToolsVersion = ext.has('buildToolsVersion') ? ext.buildToolsVersion : '27.0.3'
+def projectMinSdkVersion = ext.has('minSdkVersion') ? ext.minSdkVersion : 16
+def projectTargetSdkVersion = ext.has('targetSdkVersion') ? ext.targetSdkVersion : 27
+
 android {
-  compileSdkVersion 26
-  buildToolsVersion "26.0.1"
+  compileSdkVersion projectCompileSdkVersion
+  buildToolsVersion projectBuildToolsVersion
 
   defaultConfig {
-    minSdkVersion 16
-    targetSdkVersion 26
+    minSdkVersion projectMinSdkVersion
+    targetSdkVersion projectTargetSdkVersion
     versionCode 1
     versionName "1.0"
   }
@@ -30,5 +38,5 @@ repositories {
 }
 
 dependencies {
-  compile "com.facebook.react:react-native:+"
+  compile "com.facebook.react:react-native:${projectReactNativeVersion}"
 }