Map right-click to BACK if screen is on

Right-click was used to turn the screen on. It did nothing when the
screen was already on.

Instead, in that case, press BACK (like Vysor).

Suggested by: <https://www.reddit.com/r/Android/comments/834zmr/introducing_scrcpy_an_app_to_display_and_control/dvfueft/>

app/src/controlevent.h

@@ -20,7 +20,7 @@ enum control_event_type {
     CONTROL_EVENT_TYPE_COMMAND,
 };
 
-#define CONTROL_EVENT_COMMAND_SCREEN_ON 0
+#define CONTROL_EVENT_COMMAND_BACK_OR_SCREEN_ON 0
 
 struct control_event {
     enum control_event_type type;