Fix the default sub id returned by SmsManager.

SmsManager.getDefaultSmsSubscriptionId() did not handle the case
of no default set with a single SIM present. This CL adds the
handling for that.

Test: manual SMS sanity on single sim and DSDS devices
Bug: 132259056
Merged-in: I72ee7af9f0b2662e1ac26b7d3db60c667d06c471
Change-Id: I72ee7af9f0b2662e1ac26b7d3db60c667d06c471
(cherry picked from commit 1c0a7a16)