[2/2] Longpress back button to kill foreground app

Add a checkbox in developer options to kill
foreground app by longpressing back button.

[2/2] 长按返回键杀掉前台应用

添加一个开发者选项的勾选框, 允许长按返回键杀掉前台应用.

[1/2] in platform_frameworks_base

Thanks to CyanogenMod(@Danesh) for the original implementation.

感谢CyanogenMod(@Danesh)对此功能的原创实现.