change logic for determining emergency number

shortnumber xml will be the last resort in case there are no SIMs
(i.e. we have a list from atleast one sub)

Bug: 230284078
Test: Added unit test for this case in EmergencyNumberTrackerTest.
Change-Id: Ib0979f58c5e20f1f1edf6eba65db91e1818ab5af