Add carrier config to support local disconnect of empty IMS conference.

Adding carrier config key to support ability to locally disconnect an
empty IMS conference.  The network normally does this, but there             are some
carriers which rely on the device to do it.

Test: Add unit test to verify local disconnect when participant count is
zero.
Test: Use conference event package test cmd/intents to inject test CEP to
simulate a conference dropped to 0 participants.  Verify IMS conference is
disconnected.
Bug: 151707520
Fixes: 154245549
Merged-In: If6fb25b669a74e7e586736c04101a42ab6adcffe
Change-Id: Id7238bc668e6d6d2d7a04a8696263b260d6b6fd8