Convert PhoneAccount sort order to integers

Since it's only used to carry slot IDs, use integers instead of strings
for better clarity.

Bug: 169153499
Test: atest TelecomUnitTests:PhoneAccountRegistrarTest
Change-Id: I55f87cd9d29c8fbbd4986cc9b04d7f025df4fa33
Merged-In: I55f87cd9d29c8fbbd4986cc9b04d7f025df4fa33
2 files changed