Prevent LayoutBuilder from rebuilding more than once

[LongCatIsLooong]

Fixes #146379: introduces Element.buildScope which BuildOwner.buildScope uses to identify subtrees that need skipping (those with different BuildScopes). If Element.update calls updateChild then dirty children will still be rebuilt regardless of their build scopes.

This also introduces LayoutBuilder.applyDoubleRebuildFix migration flag which should only live for a week or less.

Caveats:

LayoutBuilder's render object calls markNeedsLayout if a descendant Element is dirty. Since markNeedsLayout also implies markNeedsPaint, the render object is going to be very repaint/relayout-happy.

Tests:

Presubmits with the migration flag set to true: https://github.com/flutter/flutter/pull/147856/checks?check_run_id=24629865893

Pre-launch Checklist

• 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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• 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.

[LongCatIsLooong]

The change moves dirty element tracking / rebuilding logic from BuildOwner to BuildScope, and keeps GlobalKey management stuff in BuildOwner