CSVT: Introduce Circuit-Switch Video Telephony.

Introduce CSVT Service which provides necessary API
for Circuit-Switch Video Telephony.

Change-Id: Ic7337939f2c7e42a47840d8f9673a9f303584707

CSVT: Add an interface to check active CSVT calls.

CSVT clients need to check if there is an active CSVT call.
Add an interface to query CSVT service if it contains an active
CSVT call.

Change-Id: Icb1d4c88009daced629070b91da0400653da4dd4
(cherry picked from commit c9c5ed960a6527ec4e0bc9962fc2096a5c307746)
(cherry picked from commit aa7acf036f9474a4254edbac4b86c02afeeaac4a)
(cherry picked from commit e230f56ad9c5397d017d7f75c24ddda07e44211d)