Create a helper lib for emergency numbers.

This util class manages getting/setting default emergency numbers,
shared by settings, EmergencyInfo and other clients.

The logic is copied from Personal Safety app - http://shortn/_Z7qtV6RpJG

Bug: 172609395
Test: manual
Change-Id: I9062d84f49556d2b7e7866c43d64dac55d48f4f4