Update build.gradle (#82)

Author: UgurGumushan

android/build.gradle

@@ -1,4 +1,3 @@
-
 buildscript {
     repositories {
         jcenter()
@@ -11,12 +10,16 @@ buildscript {
 
 apply plugin: 'com.android.library'
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 android {
-    compileSdkVersion 28
+    compileSdkVersion safeExtGet('compileSdkVersion', 28)
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 28
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 28)
         versionCode 1
         versionName "1.0"
     }