Initial call to addDisplay in DisplayModeDirector

If display is added during boot, it does not trigger onDisplayAdded during DisplayModeDirector start upon systemReady call.

This CL fixes this by explicitly calling onDisplayAdded method for all enabled displays.

Bug: 414778119
Test: atest DisplayModeDirectorTest
Flag: com.android.server.display.feature.flags.enable_on_display_added_in_observer
Change-Id: I6dfdc32db54471682ce120fe0851274ae1340f0d