[FR] - Need a setter to set FCM host for Functional tests/Service Level tests.

**Summary**:
I recently integrated FCM SDK to send Push Notifications. The integration was very simple and smooth but while I started writing Functional Tests, I got to know that I can't give FCM host while constructing a `FcmMessaging` object. The `FCM_URL` is hardcoded in the `FirebaseMessagingClientImpl` class, which is restricting me from writing functional tests.

**Solution**:
While constructing the `FcmMessaging` object or while initializing `FirebaseApp` there can be an option to pass the FCM host. If the host is not passed then the default hardcoded host must be used.

**Describe alternatives you've considered**
Currently, due to this, I'm blocked from writing FTs. I'm currently only dependent on UTs

Start by reading FirebaseMessagingClientImpl to trace how FCM_URL is set and inspect the construction paths for FcmMessaging and FirebaseApp. Done means callers can provide an FCM host for functional or service-level tests while the default hardcoded host remains effective when no host is supplied.

索引モデルが issue の本文から書いたものです。