atrace: only start trace when starting trace

Bug: 74396309
Test: atrace --async_stop > trace
Change-Id: I50a2bc224d6c48aa7cecf69a57b30c8ccc88b9e0
Merged-In: I50a2bc224d6c48aa7cecf69a57b30c8ccc88b9e0
1 file changed