Compare Emergency number display priority

Define the rules for emergency numbers that may have higher display
priority, and make Emergency number comparable for these values. This
is also essential for sorting the list so that an emergency number with
higher display priority should be listed in a lower index.

Test: Treehugger, unit test
Bug: 112657134
Change-Id: I46fb009ce862ec3c593a05b4b04e180faf198fc4