Use less proprietary libraries

## What feature would you like to see?

It's nice to see much of the SDK is open source. And it even enables the possibility to include those open source apps who use Firebase [into F-droid](https://f-droid.org/en/docs/Inclusion_Policy/), without asking them to drop Firebase in a separate build variant.

But still, the SDK depends on the proprietary `com.google.android.gms`, which prevents that from happening. Many of the usages are simply `import com.google.android.gms.tasks.Task;` `import com.google.android.gms.common.internal.Preconditions;` `import com.google.android.gms.tasks.TaskCompletionSource;` I don't know what exactly they are, but it seems many of them are just general-purpose apis, such as for concurrency.

## How would you use it?

To bring more apps to F-droid without dropping Firebase