policy: Delay FM volume update to start of FM playback

Delay update of FM volume to start of FM playback.
This avoids unnecessary set_parameter calls on primary output
whenever volume changes on primary output. Unnecessary calls
on primary output can cause breaks in audio since FastMixer
will be put to HOT_IDLE more often.

CRs-Fixed: 948361
Change-Id: Id614d32df7f35ac93dd029a08b10e19f64ac631e