Improve proto parsing layer

- Add proto's type definitions
- Reorganize repo structure to better accomodate
  the growing number of different proto versions
- Add snapshot of legacy transitions proto
  (was not added during perfetto migration)
- Simplify FakeProtoTransformer interface

Test: npm run test:presubmit
Fix: 318503129
Change-Id: Idbef2391a9b135dd9613b28f1f3d4153b1fbf72a
36 files changed