fix: only define a gradle project for module maintenance

This is the outcome of a react-native-community discussion with the goal
of avoiding downloading multiple gradle versions and/or loading multiple
versions at the same time

react-native-community/discussions-and-proposals#151 (comment)

android/build.gradle

@@ -1,11 +1,15 @@
 buildscript {
-    repositories {
-        google()
-        jcenter()
-    }
-
-    dependencies {
-        classpath 'com.android.tools.build:gradle:3.5.0'
+    // 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:3.5.0'
+        }
     }
 }