Fix network can't handle simultaneously SS request for both subs

When disabling Smart forwarding, the SS requests for both subs will be
sent simultaneously, but sometimes network can't handle both SS
requests from both subs causing call forwarding settings to not be
changed without any notification to user.

Send the next SS request after completing the previous request same as
enabling Smart forwarding.

Test: manual
Bug: 289326844
Change-Id: I4869fa200d3fbb70ba1b6b46cc491ea45299f412