Summary:
Motivation
Currently react-native run-android instals and starts your app, but if you have more than one variant, it doesn't know which one to start. This allows developers to pass in the suffix specified in build.gradle, so that the correct app is started.
Test Plan
verify that `react-native run-android` runs properly
verify that `react-native run-android --appIdSuffix validSuffix` runs properly
Closes https://github.com/facebook/react-native/pull/13169
Differential Revision: D4823391
Pulled By: ericvicenti
fbshipit-source-id: 31ed35fd79403804b4781e81eb49f1c4627d7f8e