x/mobile: Can't get the Back key event on Android

[ntop001]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.8.1 darwin/amd64

Does this issue reproduce with the latest release?

YES

What did you do?

Add following code in the basic example:

for e := range a.Events() {
	switch e := a.Filter(e).(type) {
		case key.Event:
		log.Println("key event:", e)
	}
}

Build and Run the basic example on an Android devices.

What did you expect to see?

Print the correct key code value (BacK == 4 on Android).

What did you see instead?

It prints 'KeyCodeUnknown'. It seems that in the 'android.go', there is a filter method convAndroidKeyCode which map the Back key as unknown. I think gomobile should properly process the keycode such as Back. On Android, Menu/Back are commonly used keycode. Maybe we can add a 'Origin' field in the key.Event struct, that I can get the original (no filtered) value.

PS: unity map the 'Back' key on Android to ESC.