pw_bluetooth_proxy: Reintroduce std::lock_guard

Let's confirm this change can land without issue before migrating
the rest of pw_bluetooth_proxy code back to std::lock_guard.

Change-Id: Ied2b878fc518d6c9bba280329761862a28c3daa0
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/248713
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Docs-Not-Needed: Ali Saeed <saeedali@google.com>
Pigweed-Auto-Submit: Ali Saeed <saeedali@google.com>
Reviewed-by: Ben Lawson <benlawson@google.com>
Commit-Queue: Ali Saeed <saeedali@google.com>

pw_bluetooth_proxy/acl_data_channel.cc

@@ -14,7 +14,7 @@
 
 #include "pw_bluetooth_proxy/internal/acl_data_channel.h"
 
-#include <cstdint>
+#include <mutex>
 
 #include "lib/stdcompat/utility.h"
 #include "pw_bluetooth/emboss_util.h"
@@ -26,12 +26,11 @@
 namespace pw::bluetooth::proxy {
 
 void AclDataChannel::Reset() {
-  credit_allocation_mutex_.lock();
+  std::lock_guard lock(credit_allocation_mutex_);
   initialized_ = false;
   proxy_max_le_acl_packets_ = 0;
   proxy_pending_le_acl_packets_ = 0;
   active_connections_.clear();
-  credit_allocation_mutex_.unlock();
 }
 
 template <class EventT>