Database SDK should warn when it builds databaseUrl from projectId

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

Yep

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

* Android Studio version: all
* Firebase Component: Database
* Component version: 19.6.0

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

#### Steps to reproduce:

1. Create a new project
2. Add an Android app
3. Download `google-services.json`
4. Create the default RTDB instance on the project, choose Europe location

Because (3) precedes (4) the `json` file will not contain the `firebase_url` key and therefore the RTDB SDK will try and make up the database URL:

```java
public static FirebaseDatabase getInstance(@NonNull FirebaseApp app) {
String databaseUrl = app.getOptions().getDatabaseUrl();
if (databaseUrl == null) {
if (app.getOptions().getProjectId() == null) {
throw new DatabaseException(
"Failed to get FirebaseDatabase instance: Can't determine Firebase Database URL. "
+ "Be sure to include a Project ID in your configuration.");
}
databaseUrl = "https://" + app.getOptions().getProjectId() + "-default-rtdb.firebaseio.com";
}
return getInstance(app, databaseUrl);
}
```

However it doesn't print any warning when it does this, and for anyone who chooses Europe this is the wrong URL. See developer pain here:
https://github.com/firebase/codelab-friendlychat-android/issues/137

#### Relevant Code:

See above