Make helpers_platform dep headers non public

Req #1107 (closed).

Specific-platform includes (e.g. p_thread.h) must not be included from public headers (sopc_thread.h) but only from .c files. Update CMake rules to ensure these files are not installed.

A refactor must be done to provide abstract APIs to pointers only. Some features may need to be added for some specific samples that rely on these types.