[google_sign_in] Add more serverClientId info to README

When I wrote the docs I thought that Firebase would always include a
web OAuth client ID, but it appears that this is not always the case
(e.g.,
https://stackoverflow.com/questions/55572583/google-services-json-missing-client-type-3)
so this adds more docs about what is required for this to work
correctly.

Also adds a note that a missing ID error could be due to incorrect
gradle configuration, as that's not necessarily clear from the Firebase
docs.

Fixes flutter/flutter#172073

Author: stuartmorgan-g

packages/google_sign_in/google_sign_in_android/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 7.0.3
+
+* Add more details and troubleshooting for `serverClientId` configuration
+  via Firebase.
+
 ## 7.0.2
 
 * Adds a README note about potentially confusing error returns from

packages/google_sign_in/google_sign_in_android/README.md

@@ -22,7 +22,8 @@ To use Google Sign-In, you'll need to register your application, either
 
 * If you are use the `google-services.json` file and Gradle-based registration
   system, no identifiers need to be provided in Dart when initializing the
-  `GoogleSignIn` instance when running on Android.
+  `GoogleSignIn` instance when running on Android, as long as your
+  `google-services.json` contains a web OAuth client entry.
 * If you are not using `google-services.json`, you need to pass the client
   ID of the *web* application you registered as the `serverClientId` when
   initializing the `GoogleSignIn` instance.
@@ -51,6 +52,16 @@ errors include:
 * Sign-in working in one build configuration but not another.
 
 Common sources of configuration errors include:
-* Missing or incorrect `serverClientId`.
 * Missing or incorrect signing SHA for one or more build configurations.
 * Incorrect Android package name on the server side.
+* Missing or incorrect `serverClientId`.
+
+If you are using `google-services.json` and recieve a "serverClientId must be
+provided on Android" error message, check that:
+  * Your `google-services.json` contains a web OAuth client, which should be an
+    `oauth_client` entry with `client_type: 3`. This should have been created
+    automatically when enabling Google Sign In using the Firebase console, but
+    if not (or if it was later removed), add a web app to the project and then
+    re-download `google-services.json`.
+  * You correctly added followed all of the Gradle configuration steps in the
+    Firebase integration documentation.

packages/google_sign_in/google_sign_in_android/pubspec.yaml

@@ -2,7 +2,7 @@ name: google_sign_in_android
 description: Android implementation of the google_sign_in plugin.
 repository: https://github.com/flutter/packages/tree/main/packages/google_sign_in/google_sign_in_android
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+google_sign_in%22
-version: 7.0.2
+version: 7.0.3
 
 environment:
   sdk: ^3.6.0