Allow the override of compileSdkVersion, buildToolsVersion, minSdkVersion and targetSdkVersion

Author: James Pearson

android/build.gradle

@@ -1,16 +1,20 @@
 apply plugin: 'com.android.library'
 
-def DEFAULT_COMPILE_SDK_VERSION             = 26
-def DEFAULT_BUILD_TOOLS_VERSION             = "26.0.2"
+
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 def DEFAULT_TARGET_SDK_VERSION              = 26
 
 android {
-    compileSdkVersion rootProject.hasProperty('compileSdkVersion') ? rootProject.compileSdkVersion : DEFAULT_COMPILE_SDK_VERSION
-    buildToolsVersion rootProject.hasProperty('buildToolsVersion') ? rootProject.buildToolsVersion : DEFAULT_BUILD_TOOLS_VERSION
+  compileSdkVersion safeExtGet("compileSdkVersion", 23)
+  buildToolsVersion safeExtGet("buildToolsVersion", "23.0.1")
+
 
   defaultConfig {
-    minSdkVersion 16
-    targetSdkVersion rootProject.hasProperty('targetSdkVersion') ? rootProject.targetSdkVersion : DEFAULT_TARGET_SDK_VERSION
+    minSdkVersion safeExtGet("minSdkVersion", 16)
+    targetSdkVersion safeExtGet("targetSdkVersion", 23)
     versionCode 1
     versionName "1.0"
     ndk {
@@ -21,4 +25,4 @@ android {
 
 dependencies {
   compileOnly 'com.facebook.react:react-native:+'
-}
+}