EmbeddedPkg/AndroidFastbootApp: only use ENTER or SPACE to exit

EmbeddedPkg/AndroidFastbootApp: only use ENTER or SPACE to exit

Since hotkey 'f' is used to start AndroidFastbootApp. If user
press 'f' key too long, it may be recognized pressing 'f' key
multiple times. Then AndroidFastbootApp exists since it delcares
any key press could make it exit.

So only use ENTER or SPACE key to exit AndroidFastbootApp.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>