Add settings VR listener service.

Bug: 22855417
Bug: 26724891

- Adds a settings panel allowing users to toggle the
  enabled/disabled state of each VrListenerService
  component installed.
- If disabled, a VrListenerService will not be bound
  from the framework, even when requested by a VR
  application.

Change-Id: I606eb712c011d160b9fbdd3c0e8fd744c653ef07