So far we relied on the TraceConfig.trace_uuid_msb/lsb to uniquely
identify a trace. This has several drawbacks:
1. Relies on the fact that the cmdline client sets a UUID in the TraceConfig.
   This doesn't work for other consumers who might not bother with that.
2. Assumes that the UUID never changes. This is true today (including this
   CL) but is going to change in future, when we'll need to regenerate the
   UUID for gap-less trace buffer snapshotting.

Changes of this CL:
- Introduce an explicit TraceUuid packet, which becomes the new authoritative
  source.
- If a consumer specifies a UUID in the TraceConfig, respect that.
- If a consumer doesn't, generate a random UUID in the service.

Bug: 260112703
Test: perfetto_unittests && perfetto_integrationtests
Change-Id: I4f875b14f76a5fbf1846037bd1a8eaba323b123d