Support Overwritting Gradle Values (#249)

Author: mgray2010

android/build.gradle

@@ -10,13 +10,17 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 android {
-    compileSdkVersion 26
-    buildToolsVersion "26.0.1"
+    compileSdkVersion safeExtGet("compileSdkVersion", 26)
+    buildToolsVersion safeExtGet("buildToolsVersion", "26.0.1")
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion safeExtGet("minSdkVersion", 16)
+        targetSdkVersion safeExtGet("targetSdkVersion", 22)
         versionCode 1
         versionName "1.0"
     }