Load Android Gradle Plugin conditionally (#459)

This wraps the Android Gradle plugin dependency in the buildscripts section of android/build.gradle in a conditional.
The Android Gradle plugin is only required when opening the project stand-alone, not when it is included as a dependency. By doing this, the project opens correctly in Android Studio, and it can also be consumed as a native module dependency from an application project without affecting the app project (avoiding unnecessary downloads/conflicts/etc).

for more info, you can refer to [this thread](facebook/react-native#25569) and especially [this comment.](facebook/react-native#25569 (comment)).

android/build.gradle

@@ -1,12 +1,17 @@
 
 buildscript {
-    repositories {
-        google()
-        jcenter()
-    }
-
-    dependencies {
-       classpath 'com.android.tools.build:gradle:3.3.1'
+    // 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:3.6.2")
+        }
     }
 }