Telephony: Fix "Keep preferred SMS Sim"

* Instead of checking the size of an array which is still populating (and
  on registering the first sim will obviously return a size of 1), check
  the actual sim count before resetting the user preference
* This fixes the problem that the default SIM for SMS is set to sim1 after
  each reboot

Change-Id: Ia065f2dbbd9d1b824c921fd5d2052b5eb6313aff