Allow addSubscriptionsIntoGroup with grouUuid that isn't assigned.

If a caller calls addSubscriptionsIntoGroup with a groupUuid that isn't
assigned to any subscription, we allow it. So that they don't have to
call createSubscriptionGroup if they already have a UUID value they
want to set.

Bug: 133776718
Test: manual & unittest
Change-Id: If7d9fd695fefd26722e74db98d34cd85a9333365