Move VibrationStepConductor locking out of VibrationThread.

Add thread assertions to all VibrationStepConductor methods to make
it clear where they're expected to run from. The ones not running
from VibrationThread are not expected to change or execute steps -
just to signal.

With this, we can actually reduce the locking to only lock
state that's used by multiple threads, but I'll do that in a follow-up.

Bug: 193792066
Test: presubmit, manual
Change-Id: I671517b9493ce142756a6d2544b8b4ffa5067fcb