transcoding: change getClientUids to return nullable int[]

Returning int[] in out arg list doesn't work for Java client.
Change it to return nullable int[] instead and null return
indicates failure.

Bug: 171398942
test: transcoding unit tests.
Change-Id: Ic6b8959af8785653a33697efbec075071785001b
diff --git a/services/mediatranscoding/tests/MediaTranscodingServiceTestHelper.h b/services/mediatranscoding/tests/MediaTranscodingServiceTestHelper.h
index 0d462d1..51665ea 100644
--- a/services/mediatranscoding/tests/MediaTranscodingServiceTestHelper.h
+++ b/services/mediatranscoding/tests/MediaTranscodingServiceTestHelper.h
@@ -449,13 +449,17 @@
     template <bool expectation = success>
     bool getClientUids(int32_t sessionId, std::vector<int32_t>* clientUids) {
         constexpr bool shouldSucceed = (expectation == success);
-        bool result;
-        Status status = mClient->getClientUids(sessionId, clientUids, &result);
+        std::optional<std::vector<int32_t>> aidl_return;
+        Status status = mClient->getClientUids(sessionId, &aidl_return);
 
         EXPECT_TRUE(status.isOk());
-        EXPECT_EQ(result, shouldSucceed);
+        bool success = (aidl_return != std::nullopt);
+        if (success) {
+            *clientUids = *aidl_return;
+        }
+        EXPECT_EQ(success, shouldSucceed);
 
-        return status.isOk() && (result == shouldSucceed);
+        return status.isOk() && (success == shouldSucceed);
     }
 
     int32_t mClientId;