libpdx: Add support for more event bits.

- Add generic abstraction for multiple event bits with an implementation-
  defined mechanism to deliver the bits.
- Update ServiceFS backend to pass through event bits.
- Implement EPOLLIN, EPOLLPRI, and EPOLLHUP event bit for UDS backed.

Bug: 34466748
Test: Build/flash system; observe stable operation.

Change-Id: I86afb5645b72ec69c095734c7891a690432150a3