Catch ServiceSpecificExceptions properly when getting reg state/ transport type

We were not catching ServiceSpecificExceptions that were thrown,
causing unclear exceptions. Instead, return the default value when
the ImsService is unavailable.

Bug: 192930411
Test: atest CtsTelephonyTestCases
Change-Id: I3d50b19047bc287ce6200e75bb59cf98bf8bfef0