make easy to add a checkbox preference

based from Cyanogenmod, but im adding more preferences :D
use SystemCheckBoxPreference if you want adding option for Settings.System
use SecureCheckBoxPreference if you want adding option for Settings.Secure
use GlobalCheckBoxPreference if you want adding option for Settings.Global
example:
we always using like this
    <CheckBoxPreference
        android:key="soft_back_kill_app"
        android:title="@string/soft_back_kill_app_title"
        android:summary="@string/soft_back_kill_app_summary"
        android:persistent="false"/>
and put preference into java code
but with this patch, we can use like this
    <com.android.settings.liquid.SystemCheckBoxPreference
        android:key="soft_back_kill_app_enable" << put key from android.provider.Settings
        android:title="@string/soft_back_kill_app_title"
        android:summary="@string/soft_back_kill_app_summary" />
the preference will always use presistent command
no need to add this preference into java code again :)
and say goodbye to bad java code style xD
picked from https://gerrit.omnirom.org/#/c/5759/7