audioflinger: fix failure to indicate volume to effect

If an effect is added and enabled after a track is started
it will not receive current volume information because volume
is indicated only to active effects and if there is a volume change.

Add a flag to force EffectChain::setVolume_l() to pass volume indication
to all effects each time an effect is enabled.

Bug: 17572556.

Change-Id: I6b68488c9aca8b2095fb210aa124d02e4f759660