pulseutil: Add API for setting volume ratio

pulseutil: Add API for setting volume ratio

In Tizen, a stream has one volume type such as media, notification,
alarm. Each type is so-called volume group which has its own volume
value of a certain volume level. It is managed on the Tizen policy
module of the PulseAudio. That's why the player framework has been
using volume element instead of volume property of gstpulsesink of
the pipeline to set individual volume of the stream itself.

Here we add a function to set volume ratio for the individual volume
value via the policy module of the PulseAudio which can be with the
volume group concept.

Change-Id: I6b2cf8d4814a01970e425a4dd25b49aa883ee7e1
Signed-off-by: Sangchul Lee <sc11.lee@samsung.com>