policy: Avoid unnecessary set_parameter calls

APM calls a set_parameter on primary output with key fm_volume
whenever a checkAndSetVolume is called. This is done irrespective
of whether the volume values are same of different.

A side effect of doing so is that the FastMixer associated with
the MixerThread will be put into HOT_IDLE unnecessarily which in turn
can cause glitches/breaks in audio.

CRs-Fixed: 948361
Change-Id: I7fd8b7340e6c78ee6d00b41ccf679582338c011a