Fixed problems in audio effect volume control.

Fixed the following problems in audio effect volume control in AudioFlinger:
- Make sure that the volumes returned by EffectChain::setVolume_l() are correct even is
no change is detected since last call
- Do not use isEnabled() to validate volume control but mState >= ACTIVE instead as the volume control
must be also active in STOPPING and STOPPED states.

Change-Id: Id62da3164fad500ee8a5efd6cd78c77e8fdcb541