Revert "Change SimpleStringBuilder::Append to not use strcpyn and SIZE_UNKNOWN"

This reverts commit e779847fb6499ac2dc4757de8c625ac377e9d0d4.

Reason for revert: Breaks downstream projects, depending on indirect include.

Original change's description:
> Change SimpleStringBuilder::Append to not use strcpyn and SIZE_UNKNOWN
> 
> Also add explicit includes of rtc_base/string_utils.h in files depending on it.
> 
> Bug: webrtc:6424
> Change-Id: Id6b53937ab2d185d092a5d8863018fd5f1a88e27
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135744
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Commit-Queue: Niels Moller <nisse@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#27903}

TBR=kwiberg@webrtc.org,nisse@webrtc.org

Change-Id: Ib04280d401b66fe832d3fdc9293e39276710f973
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: webrtc:6424
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/135945
Reviewed-by: Niels Moller <nisse@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#27909}

sdk/android/native_unittests/stacktrace/stacktrace_unittest.cc

@@ -19,7 +19,6 @@
 #include "rtc_base/event.h"
 #include "rtc_base/logging.h"
 #include "rtc_base/platform_thread.h"
-#include "rtc_base/string_utils.h"
 #include "rtc_base/strings/string_builder.h"
 #include "rtc_base/system/inline.h"
 #include "system_wrappers/include/sleep.h"