Better setup for android build version. (#67)

* Better setup for android build version.

React Native now has its project set up with default build version of androids sdk. Set android build version to that ofrootProjet``s to prevent from build failing cause of version mismatch.

* Safer build config for android.

Support older react-native version.

Author: hyochan

templates/android.js

@@ -16,19 +16,24 @@ buildscript {
 apply plugin: 'com.android.library'
 apply plugin: 'maven'
 
-android {
-    compileSdkVersion 23
-    buildToolsVersion "23.0.1"
+def DEFAULT_COMPILE_SDK_VERSION = 27
+def DEFAULT_BUILD_TOOLS_VERSION = "27.0.3"
+def DEFAULT_MIN_SDK_VERSION = 16
+def DEFAULT_TARGET_SDK_VERSION = 26
 
-    defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
-        versionCode 1
-        versionName "1.0"
-    }
-    lintOptions {
-        abortOnError false
-    }
+android {
+  compileSdkVersion rootProject.ext.hasProperty('compileSdkVersion') ? rootProject.ext.compileSdkVersion : DEFAULT_COMPILE_SDK_VERSION
+  buildToolsVersion rootProject.ext.hasProperty('buildToolsVersion') ? rootProject.ext.buildToolsVersion : DEFAULT_BUILD_TOOLS_VERSION
+
+  defaultConfig {
+    minSdkVersion rootProject.ext.hasProperty('minSdkVersion') ? rootProject.ext.minSdkVersion : DEFAULT_MIN_SDK_VERSION
+    targetSdkVersion rootProject.ext.hasProperty('targetSdkVersion') ? rootProject.ext.targetSdkVersion : DEFAULT_TARGET_SDK_VERSION
+    versionCode 1
+    versionName "1.0"
+  }
+  lintOptions {
+    abortOnError false
+  }
 }
 
 repositories {