Update APIs for DomainSelectionService

Update APIs per the council's review results.
1. Make DomainSelection#onBind() final
2. Make DomainSelection#getCachedExecutor() final
3. Rename EmergencyRegResult to EmergencyRegistrationResult
4. Make nullability of getters match those of setters

Bug: 324064089
Test: build
Change-Id: I483c7b37ecb9657647999dc0a65b83bb83634b4f