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

    <org.omnirom.omnigears.preference.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

Change-Id: I7da95d0db61c76ffcfc23c40dd711d322ac3bb29
Add back a helper pref that holds a system setting.

Change-Id: I7d8978829bde1f1dd15bf59f464557f64aa07beb