Move files from platform/api to platform/impl

Yuri commented on my open CL (
https://chromium-review.googlesource.com/c/openscreen/+/1791373) that 2
files currently in platform/api should be moved to platform/impl:
- platform/api/network_waiter.h
- platform/api/socket_handle.h

This CL accomplishes that, so that the attached CL can remain small

Change-Id: I7aa1b48e4c6cdfbe512a9361433b81fc8421b044
Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/1809484
Reviewed-by: Jordan Bayles <jophba@chromium.org>
Commit-Queue: Ryan Keane <rwkeane@google.com>
diff --git a/platform/impl/stream_socket.h b/platform/impl/stream_socket.h
index ec046b0..3fcade4 100644
--- a/platform/impl/stream_socket.h
+++ b/platform/impl/stream_socket.h
@@ -10,11 +10,11 @@
 #include <string>
 
 #include "platform/api/network_interface.h"
-#include "platform/api/socket_handle.h"
 #include "platform/base/error.h"
 #include "platform/base/ip_address.h"
 #include "platform/base/macros.h"
 #include "platform/base/socket_state.h"
+#include "platform/impl/socket_handle.h"
 
 namespace openscreen {
 namespace platform {