Add new Secure setting for communal mode.

This new setting will store the list of packages (apps) which the user
has enabled for communal mode.

This list is expected to change rarely, therefore we opted to store it
in Settings.Secure for simplicity as opposed to a custom file.

Test: Locally on device
Bug: 198325535
Change-Id: I9b652b5920737308da7441d324f9ef8c4d60f996

core/java/android/provider/Settings.java

@@ -10229,6 +10229,13 @@ public final class Settings {
          */
         public static final String COMMUNAL_MODE_ENABLED = "communal_mode_enabled";
 
+        /**
+         * An array of all the packages which have been enabled for hub mode by the user.
+         *
+         * @hide
+         */
+        public static final String COMMUNAL_MODE_PACKAGES = "communal_mode_packages";
+
         /**
          * These entries are considered common between the personal and the managed profile,
          * since the managed profile doesn't get to change them.