[Catalyst] Make VibrationIntensitySwitchPreference.storage val nonnull

VibrationIntensitySwitchPreference.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: Iee97b5ea8ea19fbbc0f2e870bf679dfa3fe07fc9