Add amplitude control to vibrator HAL

Providing control over the strength of the vibration allows the platform
to provide a richer haptic experience to users. How this amplitude is
modulated is left up to the vibrator implementation.

This also adds an interface to ask the HAL to perform specific haptic
effects. By exposing the intent of the haptic event to the HAL, we can
let device and haptic driver manufacturers implement custom waveforms
that more closely match the desired effect.

Test: Manual testing with Marlin HAL +
      adb shell /data/nativetest/vibrator_hidl_hal_test/vibrator_hidl_hal_test
Change-Id: Icfccb464c6c85adecdf354e2bd4cf422d7d31eb5