Introduce CLONE_SNAPSHOT for non-destructive snapshots

This CL adds the perfetto_cmd-side changes for the
non-destructive-snapshot-on-trigger feature.
The overall features works as follows:
- A new trigger mode is introduced, CLONE_SNAPSHOT.
  Semantically it's similar to STOP_TRACING but doesn't
  stop the main tracing session.
- Upon receival of a trigger, the tracing service simply
  notifies the consumer via an ObservableEvent.
- PerfettoCmd will then create another consumer instance
  in a dedicated thread and issue a --clone of the
  tracing session.

Bug: 260112703
Change-Id: Ica1307011f19cafe29a856df12d39ffc030f5503