audiomixer: add new channel volume API

audiomixer: add new channel volume API

- Add API to control left and right channel volumes.
- Fix a bug in treble gain update. The changed gain value of Treble is
  not updated in the audiomixer as the gain changed
  callback(on_eq_params_changed) doesnt look beyond bass for gain
  controls. Fix this issue by continuing  to look beyond bass control.
- Adjust the floating point comparision epsilon value from 0.000001 to
  0.00001.

Bug-AGL: SPEC-4931
Change-Id: I2a242d08f194b66abd84bb31a97364884e8d2a1d
Signed-off-by: Ashok Sidipotu <ashok.sidipotu@collabora.com>