[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

先閱讀 FirebaseMessagingClientImpl，追蹤 FCM_URL 的設定方式，並檢查 FcmMessaging 與 FirebaseApp 的建構路徑。完成的標準是：呼叫端可以為功能測試或服務層級測試提供 FCM 主機；未提供主機時，預設硬編碼的主機仍然有效。

由索引模型根據 Issue 內容生成。