Pull PopupMenuAndroid out of React Native core

Summary:
**History:** This component was originally introduced into React Native core in D52712758, to replace UIManagerModule.showPopupMenu().

**Problem:** But, React Native core should be lean. Adding this component to React Native bloats the core.

**Changes:** So, this diff pulls PopupMenuAndroid out into its own package in the react-native GitHub repository.

In the future, this will be migrated to a community package!

Changelog: [Android][Removed] Move PopupMenu out of React Native core

Reviewed By: NickGerleman

Differential Revision: D53328110

fbshipit-source-id: 469d8dc3e756c06040c72e08fa004aafa1bd6e18