WIP: Fix segfault after Unplug event for disconnected displays

On Android 10, after pressing the power button to turn off the display, hwcomposer send a hotplug event to turn off the display. The power button could be pressed again to wake up the display, but on some displays, after pressing the power button, hwcomposer send a unplug event when the display is still off, after which a segmentation fault occurs in SurfaceFlinger.

This patch added a display connection check and does not call the HandleDisplayHotplug() function, which dispatches an hotplug event when the display is disabled.