Return the existing display ids when registering a display window listener

- This ensures that the caller can synchronously initialize the existing
  displays without waiting for the onDisplayAdded() callback, which can
  happen asynchronously (since the callback is oneway).

Bug: 196186963
Test: atest WMShellUnitTests
Test: atest ActivityTaskManagerServiceTests
Change-Id: I0a8d5f9b4ede7b487a8de14bdb6eaacae7d03d9f