Reverse dependency between services and scheduler

[goderbauer]

Note to reviewers: This PR contains two commits. The first commit ( 4399765) shows what the world will look like once the migration is done. The second commit on top of that (0089896) is a temporary change that allows us to migrate customers over to the new world. Once all customers are migrated, the second commit will be reverted. I recommend reviewing the commits separately.

Description

This PR reverses the dependency between the services and scheduler layer. Prior to this PR, scheduler was depending on services. With this change it is the other way around: This allows the service layers to schedule tasks via the scheduler layer.

This work is in preparation of #6827: It will allow us to implement the state restoration manager in the service layer (the restoration manage needs access to the task scheduling API of the scheduling layer).

Breaking change: This change is expected to only break people that have custom bindings (rare). In those custom bindings, the order of SchedulerBinding and ServicesBinding will have to be switched around.

Related Issues

Pre-work for #6827.
Supersedes #54211, #54202, and #54151.

Tests

I added the following tests:

• Modified existing tests to reflect the new dependency chain.

Checklist

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• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I signed the CLA.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read Handling breaking changes.

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.
  • I wrote a design doc: https://flutter.dev/go/template Replace this with a link to your design doc's short link
  • I got input from the developer relations team, specifically from: Replace with the names of who gave advice
  • I wrote a migration guide: Migration guide for reversing dependency of scheduler/services website#3918

[goderbauer]

Changes in this file will be reverted once customers are migrated.

[goderbauer]

/cc @dnfield @Hixie

[goderbauer]

Migration guide has been posted for review: flutter/website#3918

[dnfield]

LGTM

[Hixie]

Seems reasonable.