Android cleanup: little improved way of getting SDK values

just a little improved version rather then many explicit def for each value. hence, less cluttering. also made the react-native package to get the dynamic versions as well

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

I was going to write 26 but Since we are already using latest Gradle, Therefore I wrote target values of 27 instead of 25.

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

lib/android/build.gradle

@@ -1,19 +1,17 @@
 apply plugin: 'com.android.library'
 apply from: 'gradle-maven-push.gradle'
 
-def DEFAULT_COMPILE_SDK_VERSION             = 25
-def DEFAULT_BUILD_TOOLS_VERSION             = "25.0.3"
-def DEFAULT_TARGET_SDK_VERSION              = 25
-def DEFAULT_GOOGLE_PLAY_SERVICES_VERSION    = "10.2.4"
-def DEFAULT_ANDROID_MAPS_UTILS_VERSION      = "0.5+"
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
 
 android {
-  compileSdkVersion rootProject.hasProperty('compileSdkVersion') ? rootProject.compileSdkVersion : DEFAULT_COMPILE_SDK_VERSION
-  buildToolsVersion rootProject.hasProperty('buildToolsVersion') ? rootProject.buildToolsVersion : DEFAULT_BUILD_TOOLS_VERSION
+  compileSdkVersion safeExtGet('compileSdkVersion', 27)
+  buildToolsVersion safeExtGet('buildToolsVersion', '27.0.3')
 
   defaultConfig {
-    minSdkVersion 16
-    targetSdkVersion rootProject.hasProperty('targetSdkVersion') ? rootProject.targetSdkVersion : DEFAULT_TARGET_SDK_VERSION
+    minSdkVersion safeExtGet('minSdkVersion', 16)
+    targetSdkVersion safeExtGet('targetSdkVersion', 27)
   }
 
   packagingOptions {
@@ -40,10 +38,10 @@ android {
 }
 
 dependencies {
-  def googlePlayServicesVersion = rootProject.hasProperty('googlePlayServicesVersion')  ? rootProject.googlePlayServicesVersion : DEFAULT_GOOGLE_PLAY_SERVICES_VERSION
-  def androidMapsUtilsVersion   = rootProject.hasProperty('androidMapsUtilsVersion')    ? rootProject.androidMapsUtilsVersion   : DEFAULT_ANDROID_MAPS_UTILS_VERSION
+  def googlePlayServicesVersion = safeExtGet('googlePlayServicesVersion', '10.2.4')
+  def androidMapsUtilsVersion   = safeExtGet('googlePlayServicesVersion', '0.5+')
 
-  compileOnly "com.facebook.react:react-native:+"
+  compileOnly "com.facebook.react:react-native:${safeExtGet('reactNativeVersion', '+')}"
   implementation "com.google.android.gms:play-services-base:$googlePlayServicesVersion"
   implementation "com.google.android.gms:play-services-maps:$googlePlayServicesVersion"
   implementation "com.google.maps.android:android-maps-utils:$androidMapsUtilsVersion"