[Eng] Implement web-based auth flow on Android

[data-sync-user]

We need a new AndroidAuthListener class that constructs an auth URL (same as DesktopAuthListener) and opens a new “Custom tab” with that url. After successfully signing into Mozilla accounts, Guardan can deeplink back to mozillavpn://something/<token> - AndroidAuthListener can then check AndroidActivity for the deeplink token and complete auth.

Resources:

• Android docs: Custom Tabs
• Google for Developers Blog: Upcoming security changes to Google's OAuth 2.0 authorization endpoint in embedded webviews
• Previous mobile implementation of web-based auth

┆Issue is synchronized with this Jira Task
┆Reporter: Lesley Norton