Ensure connected display ids in sysui are correct on boot

This makes the initial list of connected displays include all the ones connected already.

In this way, even after boot or sysui restarts the internal state of DisplayRepository is always coherent.

+ Fix display manager call on object construction: now getDisplays is getting called only if anyone subscribed to any flow.

+ Execute registration for display events on a background coroutine dispatcher

Bug: 298023961
Test: DisplayRepositoryTest
Change-Id: I4354314e6adcbc8b2b58e52959328d6a4a93aa69