android: refine build.gradle get versions

Use the same method in https://github.com/oney/react-native-webrtc/pull/494/files
credit: [@maxhawkins](https://github.com/maxhawkins)

And keep fallback version as default.

android/build.gradle

@@ -1,19 +1,16 @@
 apply plugin: 'com.android.library'
 
-def _ext = rootProject.ext
-
-def _compileSdkVersion = _ext.has('compileSdkVersion') ? _ext.compileSdkVersion : 27
-def _buildToolsVersion = _ext.has('buildToolsVersion') ? _ext.buildToolsVersion : '27.0.3'
-def _minSdkVersion = _ext.has('minSdkVersion') ? _ext.minSdkVersion : 16
-def _targetSdkVersion = _ext.has('targetSdkVersion') ? _ext.targetSdkVersion : 27
+def safeExtGet(prop, fallback) {
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
 
 android {
-    compileSdkVersion _compileSdkVersion
-    buildToolsVersion _buildToolsVersion
+    compileSdkVersion safeExtGet('compileSdkVersion', 23)
+    buildToolsVersion safeExtGet('buildToolsVersion', "23.0.1")
 
     defaultConfig {
-        minSdkVersion _minSdkVersion
-        targetSdkVersion _targetSdkVersion
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 22)
         versionCode 1
         versionName "1.0"
     }