Make SST.getMdnNumber() thread-safe.

Remove earlier code that was added for debugging. The main
conclusion from that was that mdn can be queried when RUIM is
not provisioned. Incorporated the fix for that in this CL.

Test: atest FrameworksTelephonyTests:ServiceStateTrackerTest#testGetMdn
Bug: 131394428
Change-Id: I490473b1c3d403a67533247c0178813c8f57edb5