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

[samtstern]

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.

[google-oss-bot]

I couldn't figure out how to label this issue, so I've labeled it for a human to triage. Hang tight.

[samtstern]

cc @mikelehen

[samtstern]

@google-oss-bot back off or I will pull your plug.

[schmidt-sebastian]

I open #266 to add these annotations to the Storage API. To do this, I created a script that can be used to get the ball rolling (the result likely needs to be modified a bit before it can be submitted). The script is here: https://gist.github.com/schmidt-sebastian/c467724506c8c3b9937118f4c6823a18

It should be called with one argument - the main folder of the source tree to modify. That way, it doesn't modify any of our test files. Since the script does not format the source afterwards, the Gradle source formatter should be run afterwards.

For now, I tried to restrict the number of annotations that I am adding. If a class isn't public, but it's methods are, I opted not to add the annotations. While this may make linting harder, it reduces the number of changes significantly. Furthermore, I am also excluding methods marked as @Override.