Document CIPD role & login for upgrading Android engine (#149433)

Uploading CIPD packages requires the `flutter-cipd-writers` role. This
simply adds this detail to the instructions for updating the Android
Engine's SDK.

This is a documentation-only change.

## Pre-launch Checklist

- [ ] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [ ] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[test-exempt]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat
[Data Driven Fixes]:
https://github.com/flutter/flutter/wiki/Data-driven-Fixes

---------

Co-authored-by: Reid Baker <hamilton.reid.baker@gmail.com>

docs/platforms/android/Upgrading-Engine's-Android-API-version.md

@@ -22,6 +22,8 @@ It is no longer recommended to upload CIPD android sdk packages manually, but if
 
 Typically, `<your-android-dir>` is in your home directory under `~/Library/Android`. The `<new-version-tag>` is what you will use to specify the new package you uploaded in the Flutter engine DEPS file.
 
+The `flutter-cipd-writers` role is required in order to complete this operation. Googlers can apply for access [here](https://grants.corp.google.com/#/grants?request=8h%2Fflutter-cipd-writers) Once this role is granted, `cipd auth-login` must be run in order for cipd to update the user's available roles.
+
 ## Engine:
 
 * DEPS: Roll buildroot hash