Change getOwnersForDomain to SortedSet

Kept the lowest to highest ordering priority, under the assumption
that less has to change in the UI/consumer if the highest priority
is popped, since it will just take it off the end without shifting
any elements. The ordering is documented.

Bug: 184891031

Test: atest DomainVerificationManagerApiTest

Change-Id: I6ea5908e356ee96aa440650f3d5025319c195266