audioflinger: fix effect volume set on start.

commit fa1e123f forced effect volume to be re applied when
an effect is enabled regardless of the fact that the volume was
already set or not on the effect chain. This caused a systematic
volume command to be send with an invalid value immediately
followed by the correct value.

Bug: 30458082
Change-Id: Ida4b5d7e96c2d4da298589c37c2ce98541a9396a