Add class comment to avoid extending ServiceState class.

ServiceState implements Parcelable and all 'Parcelables' are required to
be final, per the Android API Guidelines (go/android-api-guidelines).
This won't be changed for historical reasons, but clients should avoid
extending this class.

Bug: 128865231
Test: non-functional change that won't impact tests.
	atest CtsTelephonyTestCases:ServiceStateTest
Change-Id: Ib369f1449a3e085f404fff1c706fda0b76d42890

telephony/java/android/telephony/ServiceState.java

@@ -53,6 +53,9 @@ import java.util.stream.Collectors;
  *   <li>Operator name, short name and numeric id
  *   <li>Network selection mode
  * </ul>
+ *
+ * For historical reasons this class is not declared as final; however,
+ * it should be treated as though it were final.
  */
 public class ServiceState implements Parcelable {