Fix null pointer exception when sending SMS at short intervals

The null pointer exception was observed when sending consecutive SMS
messages at short intervals by the race condition between the
finishDomainSelection and requestDomainSelection in
SmsDispatchersController. To protect against this null pointer
exception, the requestDomainSelection which is working on the execution
flow of SmsDispatchersController is responsible for creating a
DomainSelectionConnection and the fallback to the legacy implementation
is also modified to be handled in this creation logic.
This ensures that finishDomainSelection and requestDomainSelection are
processed synchronously.

Bug: 317070933
Test: atest SmsDispatchersControllerTest
Test: manual (verified short/long SMS/Emergency SMS when domain
selection enabled - b/339313390#comment6, b/339313390#comment8)
Test: manual (verified multiple SMS/Emergency SMS (100 times) -
b/339313390#comment10 ~ comment13)

Change-Id: Idfa1a607043ed84ed8a03171ccf5f040967cb6ed