[Catalyst] Make VibrationIntensitySliderPreference.storage val nonnull

VibrationIntensitySliderPreference.storage is initialized by
`storage(Context)` method, which causes implicit sequence dependency.

NO_IFTTT=Catalyst only

Bug: 390338213
Flag: EXEMPT refactor
Test: atest
Change-Id: Icbf722c46dade8947b6f03363725f68768a1a0b2