VT: ASessionDescription: Added SDPStringFactory.

SDPStringFactory will generate SDP as string format
from the input parameters.

The generated SDP string format will be used to make
an instance of ASessionDescription.

An instance of ASessionDescription will be used
parameters of ARTPConnection to be created.

Merged-in: I73b98e41b2e4c1da699b15ed8caca98e2fea0c4f
Change-Id: I73b98e41b2e4c1da699b15ed8caca98e2fea0c4f
Signed-off-by: Byeongjo Park <bjo.park@samsung.com>