Add new ExternalVibratorService.

This lets services outside of the vibrator service (and outside of
system_server) take control of the vibrator hardware. We still apply
policy in vibrator service, however, and can ask them to mute their
operations so we can give more critical haptic feedback when necessary.

Bug: 111457573
Test: manual
Change-Id: Ib7d06aa5940790cac627499acb23e4f0bda1b035