Used to be DISABLE_ADD_CALL_DURING_VIDEO_CALL, now just
DISABLE_ADD_CALL. The meaning has also changed -- having the extra set
means that adding calls should be disabled no matter what.
Bug: 29047863
Change-Id: Icd8966492b264a9c64dc4880080a706032f0759c
(cherry picked from commit ee6e86b2bc)