rust_grpcio well known types support, default deps

The usage of the well known type Empty requires a hack in the module
above the grpc implementation, this is now the generated stem_mod.rs

This also adds additional implicit dependencies that are required by
the grpc protobuf generated code. This includes the addition of a
'header_libs' property for library dependencies which export include
paths required by protos.

We also now include both the protos and the grpcio in the library
variant via the mod_stem.rs.

Bug: 172592789
Bug: 171504899
Test: m nothing
Test: Example rust_grpcio module build command includes dependencies,
      include paths.
Change-Id: I187a13cd5cdea991828a1020314de16727e4f74e