Handle hotplug events as described instead of rescanning

Hotplug events say which display they're for and whether the display
was connected or disconnected. Before, this info was ignored, and the
event just triggered a rescan of all displays. If a display was
disconnected and then reconnected quickly, the rescan would treat this
as a no-op or a device property change and wouldn't turn the display
on.

Now the display manager attempts to update its state with the change
the event describes. So a quick disconnect/connect cycle will cause
the display to be turned on since the display manager will have
updated its internal state to reflect the disconnect event, and will
treat the connect event as a new display rather than a device property
change.

Bug: 7491120
Change-Id: Ia83f7c96b7f16f4c1bed2a73e9b56b0bf7ee3984
Process all display events in order

Display events in each batch received from IPC were being processed in
reverse order, and stopped after the first vsync event (latest
chronologically) was handled. This makes perfect sense for vsync
events, but is broken for hotplug events.

Now we process them all in order, handling all except vsync as we see
them. For vsync events, only the last is reported.

Bug: 7491120
Change-Id: I448d139d21dc27128d75ca1d661de666fff51bcb