CEC: Fix apilint errors.

- Change 'protected' to 'package private'.
- Change '@hide' to '{@hide}' for methods which should be still hidden
  for linting.
- Rename addVendorCommandListener to setVendorCommandListener and make sure to be called once.
- Fix the implementation of removeHotplugEventListener().

Bug: 18063669
Change-Id: I5c032736f17bab9518f21596f7adeac2f88ba4c1