audio_streams: Change timer start time in null_consumption_rate

```
let start = Instant::now();
```

should be moved to earlier than

```
let mut stream_source = NullShmStreamSource::new();
```

BUG=chromium:1106557
TEST=cargo test

Change-Id: Ib3af6f76fb0960ff5c844052bcd159e76d77fd6e
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/adhd/+/2303415
Reviewed-by: Cheng-Yi Chiang <cychiang@chromium.org>
Tested-by: Chih-Yang Hsia <paulhsia@chromium.org>
Commit-Queue: Chih-Yang Hsia <paulhsia@chromium.org>
1 file changed