Remove spn-conf.xml Support for SPN Override

Migrate all of the SPN override support to use CarrierConfig.
With this patch, there is now a 3 step process to
determining the displayed carrier name.
1) Check the carrier config to see if the
   CARRIER_NAME_OVERRIDE is set to true. If so, use it.
2) If there is already a non-empty SPN, use that.
3) If the SPN is empty and there is a valid
   CARRIER_NAME_STRING, use it as the fallback.

Bug: 68049890
Test: builds+boots Fi and TMO SIMs
Change-Id: I9938a5a8428368848b1616a4b59fbb72ed178c1c