Add fixed amplitude check for keyboard vibration

Make sure the keybaord vibration settings only show
up when fixed amplitude provided to avoid any unsync
behavior with platform.

Bug: 326027376
Test: atest KeyboardVibrationTogglePreferenceControllerTest
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:fd45101eee85da95a3de8d4ce2bfd01824e282c3)
Merged-In: I3b4e06660e3eaa6ce537b72831073f092cba37e2
Change-Id: I3b4e06660e3eaa6ce537b72831073f092cba37e2