Revise virtual touchpad interface.

- Explicit start and stop, outside of which the evdev devices
  don't exist.
- Permission test (not compiled by default pending build & SELinux
  support for temporarily retaining a second copy of the service
  for vr_wm).
- Enforce a single user of the touchpad.
- Support 'dumpsys'.

Bug: 36051900
Test: log inspection
Change-Id: I038ed2632d5adf50a3565a981031691d5dc5f7cd