libeffect: Fix enabled effect count calculation error

When switching from tunnel to arm effects, effect chain will be moved
from offload thread to mixer thread. During this procedure effect instance
is not destroyed, instead it's preserved and mounted onto new chain.

The migrant sequence is 1) disable instance -> 2) move to new effect
chain -> 3) enable instance again. However, the accumulated effect
enabled count is not decreased when effect instance is disabled.

As a result, LvmBundle_process() is only called once when Effect_process()
called twice, which causes input buffer get processed alternately and
glitch sound appears.

Change-Id: Idd3b8753a73694cc36617ed55b40fda93155abed
CRs-Fixed: 636353