Follow-up to the DisplayLinkManager singleton migration (#189492). FlutterMetalLayer read FlutterDisplayLinkManager.shared directly in four places when seeding and re-seeding its own CADisplayLink's preferred frame rate range.

FlutterMetalLayer can't take a constructor parameter since it's a CALayer subclass, and every instance is created implicitly by UIKit via +layerClass returning [FlutterMetalLayer class] (see rendering_api_selection.mm and the various +layerClass overrides), which always calls the parameterless -init. There's no callsite anywhere in the tree that does [[FlutterMetalLayer alloc] init] directly, so unlike VsyncWaiterIOS/FlutterKeyboardInsetManager there's no initializer to extend, and no common-initialization method to fall back on; just -init itself.

This adds a _displayLinkManager ivar, which we wire up from FlutterDisplayLinkManager.shared at the top of -init, and switches the four reads (the initial setMaxRefreshRate:forceMax: call, the maxRefreshRateEnabledOnIPhone guard, and the two re-seeds in onDisplayLink: and presentOnMainThread:) to read from it instead.

I deliberately didn't add this as a property on the class: FlutterMetalLayer.h's header comment states properties and methods "must exactly match those in the CAMetalLayer interface declaration" — it deliberately masquerades as CAMetalLayer (which you can see in -isKindOfClass:), so there's no point in increasing its API surface with this. Added the ivar in a private class extension alongside the existing _preferredDevice and _drawableSize.

No semantic change, just a refactoring, so no change to the tests.

Issue: #175879
Issue: #181684

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• 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.

If this change needs to override an active code freeze, provide a comment explaining why. The code freeze workflow can be overridden by code reviewers. See pinned issues for any active code freezes with guidance.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.