FirebaseAuth.getInstance().getCurrentUser().getUid() returning numbers in place of actual user id

### [READ] Step 1: Are you in the right place?

Issues filed here should be about bugs in __the code in this repository__.
If you have a general question, need help debugging, or fall into some
other category use one of these other channels:

* For general technical questions, post a question on [StackOverflow](http://stackoverflow.com/)
with the firebase tag.
* For general Firebase discussion, use the [firebase-talk](https://groups.google.com/forum/#!forum/firebase-talk)
google group.
* For help troubleshooting your application that does not fall under one
of the above categories, reach out to the personalized
[Firebase support channel](https://firebase.google.com/support/).

### [REQUIRED] Step 2: Describe your environment

* Android Studio version: 4.1.2
* Firebase Component: Auth
* Component version: 19.3.0 (Firebase UI 6.3.0)

### [REQUIRED] Step 3: Describe the problem

I am copying the description of this issue #1927 since I am experiencing the exact same issue.

Here is the description of #1927
"Using FirebaseAuth.getInstance().getCurrentUser().getUid() to get the user id of the user. This method is returning numbers instead of the actual user id in some case. Also, we find the users token is still valid but the user id returning from the SDK is of number example: 101528418014522522531"

I have had this reported by customers a happen a few times, and it has caused authentication to silently break for them, which is unnacceptable.

#### Steps to reproduce:

Sorry, not able to reproduce the issue and was got from Crashlytics for some users

#### Relevant Code:

```
FirebaseAuth.getInstance().getCurrentUser().getUid()
```