Unify messaging APIs

- Rename AppEvent -> WinscopeEvent.
- Move WinscopeEvent, WinscopeEventListener, WinscopeEventEmitter to src/messaging.
- Replace ad-hoc event listener/emitter interfaces with WinscopeEvent.

Fix: b/313457207
Test: npm run test:presubmit
Change-Id: I787590475e3bc2781c9e652c8452214f51a2f05f
64 files changed