Small VibrationThread fixes.

Use the correct clock, and allow the "notify" calls to be run on
VibrationThread or not (in case the HAL callback happens directly
on the thread calling into HAL), and expand commentary on this.

Bug: 193792066
Test: presubmit
Change-Id: I5039ca6e43c9220b01530fe52ea41a1ddb08f14c
2 files changed