Use RestrictTo annotations and then add a linter for all public methods

## Part 1 - Use RestrictTo annotations

If we have `public` methods that are only meant to be called by other Firebase libraries (like things in `common` or `core`) we should annotate them with `RestrictTo(Scope.LIBRARY_GROUP)`.

https://developer.android.com/reference/android/support/annotation/RestrictTo.Scope

## Part 2 - Add a lint check

Any `public` method or class in our SDKs should be annotated with one of these three annotations:

* `PublicApi` - actually meant to be used by 3p developers
* `RestricTo(Scope....)` - only meant to be used by us
* `VisibleForTesting` - not a great idea, but sometimes you need this one

## Part 3 - Abort on error

We should clean up all existing violations in the codebase and then add `lintOptions { abortOnError true }` so that we cannot footgun ourselves anymore.