Made Android version settings overridable from the main project (better future compatibility)

Author: IjzerenHein

android/build.gradle

@@ -1,3 +1,6 @@
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
 
 buildscript {
     repositories {
@@ -12,12 +15,12 @@ buildscript {
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 26
-    buildToolsVersion "25.0.3"
+    compileSdkVersion safeExtGet('compileSdkVersion', 26)
+    buildToolsVersion safeExtGet('buildToolsVersion', "25.0.3")
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 26
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 26)
         versionCode 1
         versionName "1.0"
     }