Return MDN if available from RUIM.

If RUIM is not provisioned, MDN can still be queried. However,
if RUIM is provisioned and MDN stored in SST is null, report
that as an error.

Test: atest FrameworkTelephonyTests:ServiceStateTracker#testGetMdn
Bug: 127105456
Bug: 127865360

Change-Id: I049958196671a0fd4ec7c0522c9edb9af082702f