Handle utf-8 decoding error with backslashreplace option. am: e3f93faa90 am: 3737defe14 am: 0a1132147f am: fbf95e737a am: 9b4f51a7b4

Original change: https://android-review.googlesource.com/c/platform/tools/test/openhst/+/1427008

Change-Id: If177ed9821295427937ba2e0d2f4cb6516af715b
diff --git a/stress_test.py b/stress_test.py
index 1e9bfad..d4b2323 100644
--- a/stress_test.py
+++ b/stress_test.py
@@ -282,7 +282,7 @@
               continue
 
           if self.output_fp:
-            self.output_fp.write(line.decode("utf-8").rstrip())
+            self.output_fp.write(line.decode("utf-8", "backslashreplace").rstrip())
             self.output_fp.write("\n")
 
           # Loop through all events we're watching for, to see if they occur on