Load Android Gradle Plugin conditionally

Also, reading gradle common settings from root project.

android/build.gradle

@@ -1,23 +1,32 @@
+def safeExtGet = {prop, fallback ->
+    rootProject.ext.has(prop) ? rootProject.ext.get(prop) : fallback
+}
 
 buildscript {
-    repositories {
-        jcenter()
-    }
+    // The Android Gradle plugin is only required when opening the android folder stand-alone.
+    // This avoids unnecessary downloads and potential conflicts when the library is included as a
+    // module dependency in an application project.
+    if (project == rootProject) {
+        repositories {
+            google()
+            jcenter()
+        }
 
-    dependencies {
-        classpath 'com.android.tools.build:gradle:1.3.1'
+        dependencies {
+            classpath("com.android.tools.build:gradle:3.5.3")
+        }
     }
 }
 
 apply plugin: 'com.android.library'
 
 android {
-    compileSdkVersion 23
-    buildToolsVersion '25.0.0'
+    compileSdkVersion safeExtGet('compileSdkVersion', 28)
+    buildToolsVersion safeExtGet('buildToolsVersion', '28.0.3')
 
     defaultConfig {
-        minSdkVersion 16
-        targetSdkVersion 22
+        minSdkVersion safeExtGet('minSdkVersion', 16)
+        targetSdkVersion safeExtGet('targetSdkVersion', 28)
         versionCode 1
         versionName "1.0"
     }
@@ -27,10 +36,11 @@ android {
 }
 
 repositories {
+	google()
     mavenCentral()
 }
 
 dependencies {
     compile 'com.facebook.react:react-native:+'
 }
-
+