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_coding/BUILD.gn

@@ -971,6 +971,7 @@ rtc_static_library("audio_network_adaptor") {
     "../../rtc_base:checks",
     "../../rtc_base:protobuf_utils",
     "../../rtc_base:rtc_base_approved",
+    "../../rtc_base/system:file_wrapper",
     "../../system_wrappers",
     "../../system_wrappers:field_trial_api",
   ]

modules/audio_coding/audio_network_adaptor/debug_dump_writer.h

@@ -17,7 +17,7 @@
 #include "modules/audio_coding/audio_network_adaptor/include/audio_network_adaptor.h"
 #include "rtc_base/constructormagic.h"
 #include "rtc_base/ignore_wundef.h"
-#include "system_wrappers/include/file_wrapper.h"
+#include "rtc_base/system/file_wrapper.h"
 #if WEBRTC_ENABLE_PROTOBUF
 RTC_PUSH_IGNORING_WUNDEF()
 #ifdef WEBRTC_ANDROID_PLATFORM_BUILD