audio: Add audio amplifier HAL

audio: Add audio amplifier HAL

audio: Notify amplifier HAL of device enable/disable

amplifier: add set_parameters method

- audio hal should pass the audio_device parameters to the amplifier
  to allow the amplifier to make decisions based on the additional
  parameters.  For example, we may want to change settings for DTS
  TrueMedia use case.

audio_amplifier: fix compilation

stdlib.h is needed explicitely with the new
toolchain/toolchain options.

amplifier: Fix compilation warning

* Forward declare str_parms