Add empty `io.flutter.app.FlutterApplication` to give deprecation notice, and...

Add empty `io.flutter.app.FlutterApplication` to give deprecation notice, and un-break projects that have not migrated (#164233)

The removal of the v1 embedding missed that this class was not marked as
deprecated, and did not provide a notice in the breaking changes
section. V1 apps that needed the functionality of the old
`FlutterApplication` were all broken in the previous release, but some
V2 apps have been referencing the v1 `FlutterApplication`.

For these apps, this is the same as extending the base
`android.app.Application`. So we can provide an "empty extension" of
that base class, to provide these v2 apps a deprecation notice, and
avoid breaking them without warning (in 3.29.1).

Arbitrarily chose the `spell_checker` integration test to reference this
empty application, let me know if you think I should bring up an
entirely new devicelab test instead.

## Pre-launch Checklist

- [x] 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].
- [x] I signed the [CLA].
- [ ] 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.
- [ ] 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/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#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/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md



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Co-authored-by: Gray Mackall <mackall@google.com>