Improve get SDK version values on Android (#77)

* Improve get SDK version values on Android

* Target to compile should match

Author: maicki

templates/android.js

@@ -15,18 +15,22 @@ module.exports = platform => [{
 apply plugin: 'com.android.library'
 apply plugin: 'maven'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 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
+def DEFAULT_TARGET_SDK_VERSION = 27
 
 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
+  compileSdkVersion safeExtGet('compileSdkVersion', DEFAULT_COMPILE_SDK_VERSION)
+  buildToolsVersion safeExtGet('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
+    minSdkVersion safeExtGet('minSdkVersion', DEFAULT_MIN_SDK_VERSION)
+    targetSdkVersion safeExtGet('targetSdkVersion', DEFAULT_TARGET_SDK_VERSION)
     versionCode 1
     versionName "1.0"
   }