Move file_wrapper.h to rtc_base/system/

This moves it from an API directory (system_wrappers/include/) to a
non-API directory, which is exactly what we want for utilities like
this.

BUG=webrtc:8445

Change-Id: I440974da4d347b09ff042478720d7983056b62b9
Reviewed-on: https://webrtc-review.googlesource.com/21226
Commit-Queue: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22579}

modules/audio_processing/audio_processing_impl.h

@@ -26,8 +26,8 @@
 #include "rtc_base/ignore_wundef.h"
 #include "rtc_base/protobuf_utils.h"
 #include "rtc_base/swap_queue.h"
+#include "rtc_base/system/file_wrapper.h"
 #include "rtc_base/thread_annotations.h"
-#include "system_wrappers/include/file_wrapper.h"
 
 namespace webrtc {