Emergency API review

1) Remove 'current' in Emergency APIs
2) Remove nullable
3) Handled the null Telephony service case
4) Hide getEmergencyServiceCategoryBitmask
5) Make getEmergencyUrns unmodifiable

Test: Treehugger
Bug: 122368381
Bug: 126699022
Bug: 126701211
Change-Id: I6eb1d8a3ae1625ff3f43f983c488c2969649713b