MediaTranscodingService: Simplify TranscodingClientManager.

Change the singleton to return reference instead of sp<>;

Bug: 145233472
Test: Unit test.
Change-Id: Ie5b8631ec9e917d80805f63c77618e24720f53bc
diff --git a/services/mediatranscoding/MediaTranscodingService.h b/services/mediatranscoding/MediaTranscodingService.h
index 4bdb078..cc69727 100644
--- a/services/mediatranscoding/MediaTranscodingService.h
+++ b/services/mediatranscoding/MediaTranscodingService.h
@@ -64,7 +64,7 @@
 
     mutable std::mutex mServiceLock;
 
-    sp<TranscodingClientManager> mTranscodingClientManager;
+    TranscodingClientManager& mTranscodingClientManager;
 };
 
 }  // namespace android