package com.laxture.lib;

public class PrefKeys {

    public static final String PREF_KEY_APP_TO_BACKGROUND = "app_to_background";
    public static final String PREF_KEY_APP_TO_BACKGROUND_TIME = "app_to_background_time";
    public static final String PREF_KEY_DEVICE_ID = "device_id";

}

package com.laxture.lib;

public interface PrefKeys {

    String PREF_KEY_APP_TO_BACKGROUND = "app_to_background";
    String PREF_KEY_APP_TO_BACKGROUND_TIME = "app_to_background_time";
    String PREF_KEY_DEVICE_ID = "device_id";

}

Change constant to interface