Throw exception from addServiceAnnouncement impl

Throw an UnsupportedOperationException from the base implementation of
addServiceAnnouncement and add supporting documentation.

Bug: 155926142
Test: existing CTS
Change-Id: Ib120b650d2a29acdacb9877cdeeba4b782f3f189

telephony/java/android/telephony/MbmsDownloadSession.java

@@ -453,7 +453,8 @@ public class MbmsDownloadSession implements AutoCloseable {
      * callback may include any of the errors that are not specific to the streaming use-case.
      *
      * May throw an {@link IllegalStateException} when the middleware has not yet been bound,
-     * or an {@link IllegalArgumentException} if the byte array is too large.
+     * or an {@link IllegalArgumentException} if the byte array is too large, or an
+     * {@link UnsupportedOperationException} if the middleware has not implemented this method.
      *
      * @param contents The contents of the service announcement descriptor received from the
      *                     group call server. If the size of this array is greater than the value of

telephony/java/android/telephony/mbms/vendor/MbmsDownloadServiceBase.java

@@ -234,7 +234,8 @@ public class MbmsDownloadServiceBase extends IMbmsDownloadService.Stub {
     @Override
     public @MbmsErrors.MbmsError int addServiceAnnouncement(
             int subscriptionId, @NonNull byte[] contents) {
-        return 0;
+        throw new UnsupportedOperationException("addServiceAnnouncement not supported by"
+                + " this middleware.");
     }
 
     /**