Fix leaking connection.

Issue identified by Error Prone:
Argument 'connection' should not be passed to this method; its type
ImsExternalConnection is not compatible with its collection's type argument
Integer
http://errorprone.info/bugpattern/CollectionIncompatibleType

Bug: 27723540
Change-Id: I6986ffa78bea24dd6ef5ac5ceb8b41c48edde2f5