Decouple HapticScale from ExternalVibratorService scale.

Use a conversion function rather than a static cast, and convert
to a properly typed HapticScale straight away, and all the way down to
where it's marshalled into the effect params.

Bug: 248993206
Test: presubmit, manual check scaling of audio-coupled ringtone
Change-Id: I588031ace0b3562cba50449b2147acaa6098af57