Add no-packager option to run-android and run-ios commands

Summary:
Currently, while running `react-native run-android` command, React Native's packager is launched, and there is not any way to disable the current behaviour. This is handled somehow on iOS by adding an environment variable `RCT_NO_LAUNCH_PACKAGER` (see #6180).

This is a cross-platform solution that adds the `--no-packager` option both to `run-ios` and `run-android`, which prevents the packager from being launched.

This was tested by building with and without the option, on both environments (Android and iOS) using the device and simulator, working as expected.
Closes https://github.com/facebook/react-native/pull/11735

Differential Revision: D4392170

Pulled By: ericvicenti

fbshipit-source-id: 1c31f109f18715b84cd5ab1b6d5fd758cd0a6efb

local-cli/runAndroid/runAndroid.js

@@ -31,6 +31,10 @@ function runAndroid(argv, config, args) {
     return;
   }
 
+  if (!args.packager) {
+    return buildAndRun(args);
+  }
+
   return isPackagerRunning().then(result => {
     if (result === 'running') {
       console.log(chalk.bold('JS server already running.'));
@@ -286,5 +290,8 @@ module.exports = {
     command: '--deviceId [string]',
     description: 'builds your app and starts it on a specific device/simulator with the ' +
       'given device id (listed by running "adb devices" on the command line).',
+  }, {
+    command: '--no-packager',
+    description: 'Do not launch packager while building',
   }],
 };