[input] Migrate fuchsia.ui.pointerinjector to TouchSource (#190855)

Removed fuchsia.ui.pointerinjector.Registry usages, dependencies,
manifests, and the PointerInjectorDelegate class in the Flutter Runner.
Input events for child views are routed directly by Scenic to child
TouchSources in Flatland, making parent-side pointer injection obsolete.

Test: touch-input-test, mouse-input-test, text-input-test,
flutter_runner_tests
Issue: b/42175685

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