DisplayRepository: tweak to implementation of displayAdditionEvent

- Change it so that it only emits after `displays` has emitted. This
  guarantees that calls to `getDisplay` with the newly added display id
  will always be non-null.
- This also allows us to reuse an existing `Display` instance, without
  having to do another call to `DisplayManager`

Test: DisplayRepositoryTest
Bug: 367592591
Flag: EXEMPT small implementation tweak covered with tests
Change-Id: Ic5d6f877d8ab77f13e239632f6dee1deecdf97eb