tp: keep parsing traces after wire-compatible schema changes (#6553)

When a trace embeds its own proto descriptor, trace_processor merges it
with the built-in schema. Two recent schema changes made that merge fail
on older traces, even though the trace bytes were still readable. This
tolerates both.

- Widened field type (PR #6082): previous_packet_dropped went from bool
to uint32. Both encode as a varint, so the bytes still decode. Accept a
re-declared scalar when its wire type is unchanged; only reject a real
wire-type change.

- Removed field type (PR #6308): AndroidCameraFrameEvent and
AndroidCameraSessionStats were deleted. Old traces still name them, so
resolution can't find them. Skip such a field instead of failing the
whole trace, but only for an allowlist of known-removed types so a
genuine missing type is still an error.

Both are temporary; they can go once traces predating these changes age
out (b/524094370).