UnityPackageManagerResolver helps to transition to manage packages
through Unity Package Manager, from Unity 2018.4.

- Prompts to add Game Package Registry by Google to Unity project.
  This allows the users to discover and manage packages from
  Google through Unity Package Manager.
- Added a setting window under "Assets > External Dependency Manager >
  Unity Package Manager Resolver" to add and remove registries.

Bug: 150471207
Change-Id: Ia2f1b497a412b850d561d62977808b09b7c1f5fc