Separate function to add fd to epoll

Since we will be adding video devices in addition to input devices to
epoll, we will be using the same function for adding an fd to epoll.
Small refactor here to make the later patch simpler.

Test: use booted device (EventHub native tests are pending)
Bug: 111480215
Change-Id: If257b3734ca12d0061648aa6ac5347ee1b0629a3