Add step down ramp to vibration waveforms

Update ramp-to-step adapter to also add "step down to zero" segments to
off segments in the VibrationEffect waveform. This is the step version
of the same behavior implemented for PWLEs supporting "ramp down to zero",
and uses the same configured duration.

The StepToRampAdapter no longer converts all step waveforms into PWLEs,
since now they can also smooth the abrupt amplitude transitions.

Bug: 188431691
Test: RampToStepAdapterTest
Change-Id: I6e77139f4f21d6c344617bebdb83241b7a951bde