[SettingsProvider] Add maxTargetSdk=S to ALWAYS_ON_VPN_LOCKDOWN_WHITELIST

Makes sure that third-party apps that target beyond S cannot access the
setting value directly from Settings.

BUG: 177239688
Test: manual with debug app
Change-Id: I2c5faad7f3e334704addc980530aae84c8c13864

core/java/android/provider/Settings.java

@@ -7084,7 +7084,7 @@ public final class Settings {
          *
          * @hide
          */
-        @Readable
+        @Readable(maxTargetSdk = Build.VERSION_CODES.S)
         public static final String ALWAYS_ON_VPN_LOCKDOWN_WHITELIST =
                 "always_on_vpn_lockdown_whitelist";