Fix issue 3499926.

Make sure that NumberEffectsEnabled is decremented at the same time as
SamplesToExitCountBb reaches 0 in Effect_process(). Not doing so
causes NumberEffectsEnabled not being decremented if Effect_setEnabled()
is called after SamplesToExitCountBb reaches 0 and before next Effect_process() is called.

Change-Id: Id827a301f93a5a09ecd9995a0c7d731ef526711a