Allow canPackageManageGroup for new groups

-canPackageManageGroup currently blows up (IAE) when it is
 invoked on a group that doesn't yet exist. Since the carrier
 config API method of managing groups doesn't distinguish
 between creation and updating, and since there is no
 security reason to prevent management of a nonexistent
 group, update this function to allow packages to pass the
 canManage check for nonexistent groups.

-For addSubscriptionsToGroup() apply a separate check
 to disallow adding subscriptions to a nonexistent
 group via the API (preserving current behavior).

Bug: 133442394
Test: atest SubscriptionControllerTest
Change-Id: I08600b85c6410fbbce140937132456e22291bf37