app,io/key,io/system: [API] replace system.CommandEvent with key.Event

It's much simpler to map the Android back button to a key.Event and
let the usual key filtering determine whether to block its default
behaviour.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

io/key/key.go

@@ -223,6 +223,7 @@ const (
 	NameDown           = "Down"
 	NameLeft           = "Left"
 	NameRight          = "Right"
+	NameBack           = "Back"
 )
 
 // Contain reports whether m contains all modifiers