hal: unload the modem image based on IS_ONLINE event

Timing issue is observed in which unvote of modem
from audio hal is called before the peripheral service
vote for the modem image causing modem image to unload.
Call unvote from audio hal based on the event
EVENT_PERIPH_IS_ONLINE.

Change-Id: I9fbf249707b4e4abf0708937d423e5fc41eeb9d5