Don't take the conductor lock for syncing vibrations.

The process of preparing and trigggering should be automatically
serialized and exclusive because no other step can be executing
on the VibrationThread, and only one VibrationThread is active
at any time.

Bug: 193792066
Test: presubmit
Change-Id: I93e6bdf5ea68c3602cda962fb32030c8695d5fa8