Move TOS/PP to Auth Method Picker screen, provide guidance for single-provider flow

[samtstern]

This is an extension of #492 based on some internal discussion about the best way to handle this.

Currently we show the TOS/PP in two places:

• Email sign up screen
• SMS code prompt screen

Some problems:

• Before getting to the email sign up screen, you have already entered some personal information (in the check email screen)
• Sometimes the SMS code prompt screen is skipped when auto-verification works
• There is no TOS/PP shown for social flows
• When you enter a single-provider flow, there's no FirebaseUI-owned UI at all

Solutions:

• Remove TOS/PP from everywhere that it is present
• Add TOS/PP on the AuthMethodPicker screen, which is shown before any multi-provider flow
• Add guidance in the README that developers using FirebaseUI for single-provider flows should show a TOS/PP screen in their own UI before launching FirebaseUI as there's no guarantee we will have a chance

[samtstern]

Here are some examples of how other auth UI frameworks do this.

Okta

Auth0

Personally I think the Auth0 approach is pretty good.

[samtstern]

Update:
In the case of the email/phone flows we will continue to show TOS/PP with the following changes:

• Move it to the first screen in each flow
• Only show it in the single-provider case, which means the user will not have seen TOS/PP links in the Auth Method Picker screen

We will still need to provide guidance to developers using email flow because SmartLock could cause the UI to be completely skipped.

[morgler]

Are you aware of the GDPR law requiring explicit consent of the user (e.g. by checking a checkbox)? Just saying "By signing up, you agree" is not legal anymore in any EU country.

More on this law: https://www.eugdpr.org

[eliotstock]

16 days till GDPR, and apps aren't compliant without this. Can we get an ETA please?

[samtstern]

The fix for this has been merged into 4.0.0, now just waiting for translations to come through. Committed to getting this released before the deadline.

[ahaverty]

@samtstern Have firebase any thoughts on the "requiring explicit consent" condition/ @morgler 's comment and whether the upcoming v4.0.0 changes are actually compliant?

[samtstern]

We believe these changes are compliant.

[samtstern]

This has been fixed and released in 4.0.0