VT: Add fn that decides ratio of actual Enc bitrate from TMMBR

[Problem] VT downgrades automatically in long VT call
[Cause] In some place, NW drops pkts continuous & periodically.
  Because drop is continuous, TMMBR is always around
  min bitrate that carrier permits.
  By a bad chance, VT is downgraded if actual bitrate
  is under the min continuously.
[Solution] The ratio of actual Enc bitrate from TMMBR
  is too much in low bitrate situation.
  So add setParam to adjust the ratio from SVC app.

Bug: 419107865
Test: Ran presubmit and existing unit test passed.
Flag: EXEMPT bugfix

Change-Id: I55a73e9f8f4b7e4163cba51939cfc81de7c307e8
Signed-off-by: Kim Sungyeon <sy85.kim@samsung.com>