Summary:
This diff implements iOS-specific `PlatformRunLoopObserver` (and `MainRunLoopObserver`) that then being glued together with `SynchronousEventBeat` replaces `MainRunLoopEventBeat`, and then same thing glued with `AsynchronousEventBeat` replaces `RuntimeEventBeat`.
So, instead of two platform-specific classes we had on iOS (for that needs), now we have only one (that can be reused for a more broad variety of applications).
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: sammy-SC
Differential Revision: D21341998
fbshipit-source-id: fafde4e678770f7fcf9c1ff87acc02812a37e708
Valentin Shergin