[firebase_messaging] Update various build dependencies (firebase#259)

* Bump Gradle & AGP to latest versions.

* Bump google-services dependency to 4.3.2

[firebase_auth_web] Removed onMethodCall override and related imports. (

firebase#1630)

Removed onMethodCall override and related imports.

[firebase_auth_web] Removed onMethodCall override and related imports. (

firebase#1630)

Removed onMethodCall override and related imports.

Fix analysis failures in cloud_functions (firebase#1626)

* Fix analysis failures

[firebase_storage] Added error handling to writeToFile (firebase#1482)

* [firebase_storage] Added error handling to writeToFile (StorageFileDownloadTask) - see firebase#1418.

* Added error handling for integration testing of writeToFile.

Fix pubspec format (firebase#1599)

Fix pubspec format (firebase#1599)

[firebase_auth] Endorse web implementation (firebase#1601)

Fix pubspec format (firebase#1599)

[firebase_admob] Support v2 embedding (firebase#1365)