Make VibrationThread long-lived.

This CL changes VibrationManagerService/VibrationThread to create
and re-use a single thread instance, rather than spawning and starting
a new thread for every vibration.

There is still only ever one active vibration at a time.

Bug: 193792066, 219776664
Test: manual
Change-Id: I9d397ff32a174f8e02c8a490b2b3d4ea040650db
6 files changed