pw_system: Move snapshot proto call to namespace

Fix call to ThreadSnapshotService to include the .proto namespace.

Bug: b/289376657

Test: device.snapshot_peak_stack_usage() from pw_console works on
      stm32f429i

Change-Id: I7ce8bbb7aec957899447f8c2b22a2a5942348344
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/153651
Reviewed-by: Armando Montanez <amontanez@google.com>
Presubmit-Verified: CQ Bot Account <pigweed-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Andres Oportus <andresoportus@google.com>

pw_system/py/pw_system/device.py

@@ -157,9 +157,8 @@ def print_metrics(metrics, path):
         return metrics
 
     def snapshot_peak_stack_usage(self, thread_name: Optional[str] = None):
-        _, rsp = self.rpcs.pw.thread.ThreadSnapshotService.GetPeakStackUsage(
-            name=thread_name
-        )
+        snapshot_service = self.rpcs.pw.thread.proto.ThreadSnapshotService
+        _, rsp = snapshot_service.GetPeakStackUsage(name=thread_name)
 
         thread_info = thread_pb2.SnapshotThreadInfo()
         for thread_info_block in rsp: