Constrain scaled amplitude to valid values in [1,255]

Scaling down low amplitudes can lead to one-shot effects with amplitude
zero, which leads to an invalid VibrationEffect and causes
IllegalArgumentException to be thrown on an originally valid effect.

Bug: 179674498
Test: VibrationEffectTest
Change-Id: Ie653084d37f9bfed20aac25f8805ffdf40661e03