SurfaceFlinger: fix dispatching DISPLAY_EVENT_CONFIG_CHANGED

When setting a new desired config, we might be in the middle of a
previous config changed. We need to maintain the information about
whether we need to generate a config changed event and dispatch it when
we are done with the config switch.

Test: manual test to change config from SF
Bug: 122905403
Change-Id: I7efad63b666e83f6459a65a40e6cc880119ca7f6