Add documentation for GoogleApiHelper#getSafeAutoManageId() and do a teeny bit of cleanup in the auth README

[SUPERCILEX]

@samtstern I recently added Firebase app indexing to my app and noticed that it was crashing if I tried to sign in or out. Turns out I was duplicating enableAutoManage ids with FirebaseUI so I think we should make GoogleApiHelper#getSafeAutoManageId() part of the public api so people can use it to get around this problem.

[SUPERCILEX]

This header is part of the Table of Contents so it should be on the same level as configuration and using-firebaseui-for-authentication.

[SUPERCILEX]

@samtstern I think the api is stable enough for this, do you agree?

[samtstern]

Hey @SUPERCILEX I don't think this should be part of the public API for a few reasons:

1. It's tangential to our library's core function, so if this was a pattern we wanted people to use we'd do better to just publish a snippet somewhere (Github Gist, StackOverflow, etc)
2. It's really just wrapping AtomicInteger, so we're not providing much value
3. The GoogleApiClient engineers made a decision to not just auto-increment IDs internally so that users would have to track their clients, so while for us this pattern works it's not something we want everyone to do without thinking.

Gonna close this one since the other README fixes can be part of another PR, hope that makes sense!

[SUPERCILEX]

@samtstern your arguments totally make sense on not making it part of the public API, but as a counter proposal I think we should at least start the atomic integer with a value of 10 or something. What do you think? This should remove the need for devs to use getSafeAutoManageId() because we don't start with the default ID anymore. Also, I didn't think about this but we weren't using auto manage before so this is technically breaking backwards compatibility.