Suspend instead of stop for media.STOP client OP.

Stop means to stop playback and reset the session with no intention to resume
it. To allow resume after stopping, use the suspend API instead.

Bug: b/140945356
Test: Manual test
Change-Id: Ib4b76aa64d7ed1d805c0c4761375faf2c6c447b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2158046
Commit-Queue: Li Lin <llin@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761006}

chromeos/services/assistant/assistant_manager_service_impl.cc

@@ -912,7 +912,7 @@ void AssistantManagerServiceImpl::OnMediaControlAction(
   }
 
   if (action_name == kStopTrackClientOp) {
-    media_controller_->Stop();
+    media_controller_->Suspend();
     return;
   }
   // TODO(llin): Handle media.SEEK_RELATIVE.