gen_amalgamated: Move cpp proto bindings from perfetto.cc to perfetto.h

Since some of the tracing SDK functionality depends on cpp protobuf
messages (e.g., track descriptors), move the definitions of these into
the public perfetto.h header from the private perfetto.cc
implementation. This ensures the full binding definitions are available
for embedders to use.

We also add some code to the SDK example that exercises the cpp
bindings.

Change-Id: Ife88c5fbca81ab1e5b6dfac2ff537e36b12b40a8