Prep for alignment with Flutter analysis options

[stuartmorgan-g]

Renames the old analysis_options.yaml to analysis_options_legacy.yaml,
replacing it with a slightly modified copy of flutter/flutter's analysis
options.

Each plugins has a temporary local analysis_options.yaml that points to
the legacy version. This allows for inceremental conversion on a
per-plugin basis, which should make the problem more tractable.

Since this hasn't yet been enabled for any packages, it's likely that as
it is we'll find a few local modification we need to make to the root
analysis_options (e.g., things that conflict with 'dart format').

Part of flutter/flutter#76229

Pre-launch Checklist

• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

[ditman]

This change is breaking the rolls into flutter/flutter:

Activated flutter_plugin_tools 0.0.45+3.
▌09:47:03▐ ELAPSED TIME: 5.143s for ../../../k/flutter/bin/cache/dart-sdk/bin/pub global activate flutter_plugin_tools in ../../tmp/t/pluginsDYZTNQ
▌09:47:03▐ RUNNING: cd ../../tmp/t/pluginsDYZTNQ; ../../../k/flutter/bin/cache/dart-sdk/bin/pub global run flutter_plugin_tools analyze
Verifying analysis settings...
Found an extra analysis_options.yaml in /b/s/w/ir/tmp/t/pluginsDYZTNQ/packages/android_alarm_manager/analysis_options.yaml.
If this was deliberate, pass the package to the analyze command with the --custom-analysis flag and try again.

We should let the analysis command that all our plugins have a custom analysis_options.yaml for now?

Investigating.

[ditman]

The change needs to happen around here:

https://github.com/flutter/flutter/blob/2e06ed078726e31c5afca36ae9c6736060c7b7e8/dev/bots/test.dart#L851-L860