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

cc/linkable.go

@@ -64,6 +64,10 @@ func StaticDepTag() blueprint.DependencyTag {
 	return libraryDependencyTag{Kind: staticLibraryDependency}
 }
 
+func HeaderDepTag() blueprint.DependencyTag {
+	return libraryDependencyTag{Kind: headerLibraryDependency}
+}
+
 type SharedLibraryInfo struct {
 	SharedLibrary           android.Path
 	UnstrippedSharedLibrary android.Path