add ram bundle to android template (#24264)

Summary:
I plan to do this for a long time :)
Still a little surprised so much people not know it https://twitter.com/geekykaran/status/1112756026611257344

[Android] [Feature] - add ram bundle to android template
Pull Request resolved: https://github.com/facebook/react-native/pull/24264

Differential Revision: D14725396

Pulled By: cpojer

fbshipit-source-id: b8e424f0be81978465259c82024f206d232acb55

template/android/app/build.gradle

@@ -18,6 +18,9 @@ import com.android.build.OutputFile
  *   // the entry file for bundle generation
  *   entryFile: "index.android.js",
  *
+ *   // https://facebook.github.io/react-native/docs/performance#enable-the-ram-format
+ *   bundleCommand: "ram-bundle",
+ *
  *   // whether to bundle JS and assets in debug mode
  *   bundleInDebug: false,
  *