[Chromecast] Add Display Brightness Changed Event

Adds a new DISPLAY_BRIGHTNESS_CHANGED value to AvSettings's Event enum.

This event should be fired when display brightness is changed.

BUG=internal b/76421244
BUG=internal b/77479163
TEST=None

Change-Id: Ifd9661c63fb5684e04358b5a44e2035bee87c10c
Reviewed-on: https://chromium-review.googlesource.com/1045311Reviewed-by: Stephen Lanham <slan@chromium.org>
Reviewed-by: Sergey Volk <servolk@chromium.org>
Commit-Queue: Fergus Simpson <afergs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#557065}

chromecast/public/avsettings.h

@@ -136,6 +136,9 @@ class AvSettings {
 
     // This event should be fired when an HDMI error occurs.
     HDMI_ERROR = 102,
+
+    // This event should be fired when the display brightness is changed.
+    DISPLAY_BRIGHTNESS_CHANGED = 200,
   };
 
   // Delegate to inform the caller events. As a subclass of TaskRunner,