update build.gradle

Author: jamieg

android/build.gradle

@@ -13,12 +13,17 @@ buildscript {
     }
 }
 
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
+
 android {
-    compileSdkVersion 27
-    buildToolsVersion "27.0.3"
+    compileSdkVersion safeExtGet("compileSdkVersion", 28)
+    buildToolsVersion safeExtGet("buildToolsVersion", "28.0.3")
+
     defaultConfig {
-        minSdkVersion 23
-        targetSdkVersion 26
+        minSdkVersion safeExtGet("minSdkVersion", 23)
+        targetSdkVersion safeExtGet("targetSdkVersion", 27)
         versionCode 1
         versionName "1.0"
     }