Enable smoother signin if the user is signed in to Firefox on mobile

[rfk]

If the user is already signed in to Firefox on their mobile device, it should be possible to streamline the process of signing in to the Send app on the same device. Fennec provides an "AuthProvider" interface that allows it to share its login state out to other apps, see e.g. how Lockwise integrated with it here:

mozilla-lockwise/lockwise-android#879

Please consider adding something similar to the Send mobile app.

Note that I don't think it's possible to automagically sign in to Send using the data that's stored in Fennec, because Send uses a distinct encryption key from the one that Fennec holds for Sync. But it should be possible to at least pre-fill the user's email address and perhaps avoid asking for email confirmation or 2FA.

[rfk]

/cc @grigoryk

[grigoryk]

This would need a change to Fennec, adding Send's appid&signature to the whitelist of allowed applications. Lockwise was whitelisted from day one, so it was easy to land that functionality there once it became available in a-c.

Instead of adding Fennec support for this, I'd refocus this bug around having Fenix act as an AuthProvider. Fennec is a dead-end here, but we can build this in a forward-looking way.

Work that will enable Fenix side of this functionality is tracked in mozilla-mobile/android-components#2776