Fix ConcurrentModificationException getting emergency nums

Telephony API getting emergency number for category sometimes causes
ConcurrentModificationException. This change filters the emergency
number category by itself instead of relying on the buggy API.

There is a separeate bug tracking fix the API at the root.

Fix: 174635447
Test: manual
Change-Id: I88526bc0bdc22993678fcf64ecec455a219bdaaf