GitLab

- Add exo::Gamepad interface.
- Add exo::GamepadObserver interface.
- Add WaylandGamepadVibratorImpl to handle vibration and cancel
vibration requests from the client.
- Refactor WaylandGamingSeatDelegate so that WaylandGamepadDelegate
handles all gamepad-specific events.
- Add Wayland protocol to support gamepad rumble. The zcr_gamepad_vibrator_v2 interface is created when the server calls
the vibrator_added event. The client can then trigger/disable vibration
events on a gamepad vibrator through the interface.

BUG=b:158686809
TEST=Add unit tests in gamepad_unittest. All tests pass in
exo_unittests.

Change-Id: Ie4b1b3b5f9748f31846c54c97cb4f7c315d09a05
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2313976
Commit-Queue: Merissa Tan <liyingtan@google.com>
Auto-Submit: Merissa Tan <liyingtan@google.com>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#798253}