Add support to visit login status

app/src/main/java/com/hippo/ehviewer/Settings.java

@@ -50,6 +50,8 @@ public class Settings {
      ********************/
 
     public static final String KEY_THEME = "theme";
+
+    public static final String KEY_ACCOUNT = "account";
     public static final String KEY_BLACK_DARK_THEME = "black_dark_theme";
     public static final int THEME_LIGHT = 1;
     public static final int THEME_SYSTEM = -1;
@@ -434,6 +436,7 @@ public class Settings {
         putInt(KEY_VERSION_CODE, value);
     }
 
+    @Nullable
     public static String getDisplayName() {
         return getString(KEY_DISPLAY_NAME, DEFAULT_DISPLAY_NAME);
     }