Rename ObjC plugins to have `FLT` prefix

Per [recommended conventions in Objective C programming](https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/ProgrammingWithObjectiveC/Conventions/Conventions.html), our classes should have a 3-letter prefix that uniquely identifies their namespace.

e.g. `FLTUrlLauncherPlugin`, etc.