InputFlinger: Support amplitude control for InputDeviceVibrator

Add support for sending multi-channel rumble amplitudes to input
devices supporting FF_RUMBLE.

Bug: 38511270
Bug: 136215622
Test: Connect a gamepad whose driver supports FF_RUMBLE, find it
      with the android input framework, and do something like this:

      // waveform where rumble magnitude doubles every 2 seconds
      VibrationEffect effect = VibrationEffect.createWaveform(
              new long[] { 2000L, 2000L, 2000L, 2000L, 2000L },
              new int[] { 16, 32, 64, 128, 255 },
              -1);
      inputDevice.getVibrator().vibrate(effect);

Change-Id: I2f059e085c106cbca2372c72d09a9f579d35e4c7