[iOS] Mark DisplayLinkManager.shared and init() @MainActor - #189815

Now that DisplayLinkManager is injected in nearly all use-cases and the .shared singleton is used at exactly 3 remaining call-sites, marks .shared and init() as @MainActor now that it's safe to do so.

DisplayLinkManager.init() has always required betin called on the main thread since it reads UIScreen.main/Bundle.main, but that was only enforced by a runtime assert. which compiles out in most release configruations. The Swift compiler can now enforce a static, checkable contract instead. Actor-isolation checking is enforced in Swift 5.5+ regardless of strict-concurrency level or language mode, so this takes should take effect immediately given our current Swift 5 language mode/default concurrency checking not gated behind a migration.

I've left the manual assert(Thread.isMainThread, ...) in place for now given the existing Obj-C use in FlutterViewController and FlutterMetalLayer. If we do eventually remove it, we should ensure that the runtime itself is enforcing this runtime behaviour first -- this is dependent on SDK version.

I've also left the internal testing initializer non-isolated: it doesn't touch UIScreen/Bundle.main, so it has no main-thread requirement, and leaving it alone keeps the two tests that use it compiling unchanged.

DisplayLinkManagerTest.swift's testSharedInstanceReturnsAValidValue and testDisplayConfigurationNotificationsAreHandledWithoutCrashing both call .shared directly, so we needed to mark the test class @MainActor. XCTest already runs these on the main thread, so this is a no-op but makes the compiler (which doesn't know about XCTest's implementation) happy.

No semantic changes to tests because there's no semantic change to the logic, and this is enforced by the complier itself.

Issue: #175879
Issue: #181684

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