[1/2] Navigation bar buttons customization

* Add option for user to customize navigation bar.
* Add device type checking using the previously added utility function.
* Fix buttons layout in mdpi & hdpi in landscape.
* Fix search_light pointing direction in landscape.
* Fix issues with RTL layouts.
* Fix orientation determination logic.
* Add alpha value applied to all layouts.
* Fix error with backAlpha values.
* Add null check all button usages.

[1/2] 个性化导航栏按键

* 添加导航栏自定义的选项.
* 添加用先前加入的功能对设备类型进行检测.
* 修复mdpi和hdpi在水平视图下的按键布局.
* 修复search_light在水平视图下的指向.
* 修复RTL布局下的一些问题.
* 修复旋转方向判定逻辑.
* 添加alpha值至全部布局.
* 修复backAlpha值错误, 采用findButton而不是findViewWithTag.
* 添加按键使用的空值检测, 用户可能隐藏他们.

Thanks to CyanogenMod for the original implementation.
感谢CyanogenMod原创实现此功能.

[2/2] in platform_packages_apps_Settings