[2/2] User configurable low battery indicator

Add a user configurable option which allow users to choose
from the following low battery indicator methods.

* Show dialog + play sound (default)
* Fire notification + play sound
* Show dialog only
* Fire notification only
* Play sound only
* Do nothing

[2/2] 用户可配置的低电量提示

添加一个用户可配置的选项来允许用户从下列低电量提示方式中选择.

* 显示弹窗 + 播放声音 (默认)
* 触发通知 + 播放声音
* 仅显示弹窗
* 仅触发通知
* 仅播放声音
* 神马都木有

[1/2] in platform_frameworks_base