[aapt2] Fix and optimize tracing

- Make it log the tags for both begin and end events, otherwise
  visualizations can't calculate the duration

- Wrap the whole file in a JSON array to make Perfetto open it

- Switch the whole thing off when not enabled, don't waste
  cycles on adding events.

Bug: 294025081
Test: manual tracing run
Change-Id: I42d0fa5518190783c4d58a644f783ac6e7dcd1a3