Move VibratorHelper to a single threaded executor

VibratorHelper has been using the @Background Executor, but the
background thread has often been saturated.
This causes vibrations to be delayed.

Ideally we need a oneway call to Vibrator, that doesn't block our main
thread, but until then we'll use a single thread Executor.

Bug: 245528624
Test: manual
Test: atest VibratorHelperTest
Change-Id: I7349c24c1c6ed5cfa658e7055126103f4d16dee6