tp: simplify JSON trace tokenization

* fold metadata and process dump into fallthrough key handling - they
  were ignored anyway, just treat them as keys we know nothing about.
* add support for long json string/dicts in fallthrough handling: needed
  because of previous point.
* stop using next + 1 without justification - instead verify the
  value starts how we expect and explicitly increment.
* make ReadOneJsonString expect to see the first quote instead of
  starting after it: goes with previous point

Change-Id: I53832f63d96c0396712f33d3a63d121626e05287
Fixes: https://github.com/google/perfetto/issues/350