traceconv: ignore userspace events for traces with Android data
traceconv: ignore userspace events for traces with Android data

This CL changes traceconv to ignore the JSON codepath in the situation
where the raw table has non-Chrome events. This allows for both Chrome
developers looking at Chrome-only traces and Android developers looking
at Android-only traces to work.

We still need to figure out what to do for merged traces but this
requires a lot more thought.

Bug: 142902430
Change-Id: I3035e3115220e4835b97838dbba76ebb9c5e563d
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto