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