Several improvements in audio effects volume control.

- Fixed crash when deleting an effect chained before an effect having volume control
- Changed EFFECT_FLAG_VOLUME_CTRL to implicitely include EFFECT_FLAG_VOLUME_IND
(not need to set both in effect descriptor).
- Volume control changes from one effect to another if needed according to effect enable state
- EFFECT_CMD_SET_VOLUME is only sent when their is an actual change in volume

Change-Id: Ieebaf09157e2627366023569d95516646e03e26c