fix: Android stand-alone build.gradle with .gitignore (#33)

.gitignore

 .gradle
 local.properties
 *.iml
+android/gradle
+android/gradlew*
 
 # BUCK
 buck-out/

android/build.gradle

   return rootProject.ext.has(name) ? rootProject.ext.get(name) : defaultVaue
 }
 
+buildscript {
+    // 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'
+        }
+    }
+}
+
 apply plugin: 'com.android.library'
 
 android {
   }
 }
 
+repositories {
+    google()
+    maven {
+        // All of React Native (JS, Obj-C sources, Android binaries) is installed from npm
+        url "$rootDir/../node_modules/react-native/android"
+    }
+    jcenter()
+}
+
 dependencies {
   //noinspection GradleDynamicVersion
   implementation 'com.facebook.react:react-native:+'