Add an AuthUI.reauthenticate() function

[samtstern]

There are some Firebase Auth actions that require a recent (~5mins) sign-in to succeed:

• Delete account (AuthUI.delete() does not handle reauthenticate #478)
• Change email
• Etc

We should offer a simple method to launch into a re-authentication flow that allows the user to sign in choosing from any of their linked providers.

Considerations:

• This would likely have to be a method that returns an Intent similar to the sign-in intent
• The developer will probably want to provide a reason for the re-authentication, maybe passed in
• This would be similar to the normal sign-in flow with a few alterations:
  • Account creation disabled
  • Account picker screen lists only methods the user has already linked to the FirebaseUser

[amandle]

Unless there are objections I'm going to start taking a crack at this.

[samtstern]

Go for it, thanks!