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}
diff --git a/modules/audio_device/BUILD.gn b/modules/audio_device/BUILD.gn
index 6cc9b20..bfa095b 100644
--- a/modules/audio_device/BUILD.gn
+++ b/modules/audio_device/BUILD.gn
@@ -187,6 +187,7 @@
     "../../rtc_base:deprecation",
     "../../rtc_base:rtc_base_approved",
     "../../rtc_base:rtc_task_queue",
+    "../../rtc_base/system:file_wrapper",
     "../../system_wrappers",
     "../../system_wrappers:metrics_api",
     "../utility",