Fix startLogin being called on every configuration change with only one provider enabled

[SUPERCILEX]

#549

[samtstern]

LGTM, but can you quickly explain how to reproduce the intial bug? I didn't really understand the report in #549

[SUPERCILEX]

@samtstern Of course!

Using a single provider with or without smart lock eventually calls redirectToIdpSignIn -> an instance of IdpSignInContainer is created with setRetainInstance(false) (by default) -> on every screen rotation, startLogin gets called. The reason we don't see this bug anywhere else in FirebaseUI is because stuff like AuthMethodPickerActivity doesn't use IdpSignInContainer. IdpSignInContainer's only usage is for SignInDelegate.

[SUPERCILEX]

@samtstern I just realized you might have been asking for a non-technical explanation for which the answer would be: start login with a single provider -> get to the provider's login section (eg not the smart lock thing) -> rotate the screen.

[samtstern]

@SUPERCILEX yep the second one was what I wanted, confirmed the bug.

[samtstern]

@SUPERCILEX https://travis-ci.org/firebase/FirebaseUI-Android looks like the build is failing with a lint error here but I don't get any on my machine. Can you reproduce?

[SUPERCILEX]

@samtstern Travis says it's because support lib 25.1.1 came out https://developer.android.com/topic/libraries/support-library/revisions.html#25-1-1

I'll create an update PR later today since it's just bug fixes (and I'll also disable GradleDependency to make our builds less flaky like in #542).