Fix MQ tracing for postSyncBarriers (legacy)

Ie097615d489f05a8e838f8618fca2216d9665373 fixed
double counting for postSyncBarriers for concurrent
message queues but broke it for legacy.

In the legacy queue, we don't enqueue the message as part of postSyncBarrier so removing the increment on the queue
undercounted the legacy queue depth leading to negative depths.

Test: Manual
Flag: android.os.perfetto_sdk_tracing_v2
Bug: 303199244
Change-Id: I7f54ed38948b81d0ebc3cb86c2165d1f68ea9014