Runtime multidsplay handling through virtio gpu

Bug: 184849633

* Display change listener
* Add/Del/Modify secondary displays at runtime. When hotplug is called
with DISCONNECTED, surfaceFlinger will call hwc2 to destroy layer, stop
vsync.

Test: add/modify/del display at runtime

Change-Id: Ia8a1230c615ef6359213d6297d43dfd5eec37bc2