EDID support for external display hotplug

The hardware_composer caches the edid when a new hotplug event fires
(connected and disconnected). The display services uses this cached edid
when the edid is requested.

Bug: 141007548
Test: Manual, ran on mtp device with wave optics plugged in and
unplugged and verified edid with: "adb shell dumpsys SurfaceFlinger --display-id"

Change-Id: I4556e1d8a3f8677b1d9301d9afbc40d514c86e27