logcat_parsing: Replace errors when decoding logcat output

Some devices print non standard characters to logcat. If an error
occurs when parsing the output, replace the offending character instead
of raising an error.

wa/workloads/jankbench/__init__.py

@@ -153,7 +153,7 @@ class Jankbench(ApkWorkload):
     def extract_metrics_from_logcat(self, context):
         metric_names = ['mean', 'junk_p', 'std_dev', 'count_bad', 'count_junk']
         logcat_file = context.get_artifact_path('logcat')
-        with open(logcat_file) as fh:
+        with open(logcat_file, errors='replace') as fh:
             run_tests = copy(self.test_ids or self.valid_test_ids)
             current_iter = None
             current_test = None