Add logging to file for lite interpreter

Summary:
Implement file logging for lite interpreter

Notes on a few changes:
* `log_screen` - a race condition between `self.util.run` would cause the logs to be overwritten despite being read earlier. We don't necessarily need the results of this for xcode, so we simply overwrite this with the logfile contents
Weird condition with local testing:
* stdout logs don't show up if the benchmark is run while the phone is unlocked. See https://www.internalfb.com/intern/aibench/details/389282756977192 for example.
* Logs only show up if the phone is locked at the start of the benchmark, then is manually unlocked after the app is installed https://www.internalfb.com/intern/aibench/details/769472202389124
* Problem does not reproduce on prod devices, for example on iPad: https://www.internalfb.com/intern/aibench/details/945345102000109

Reviewed By: Jack-Khuu

Differential Revision: D62042318

fbshipit-source-id: 9acd32860be1633ade8b1cac11cf91d2ae10da5f

Author: Varun Puri

benchmarking/platforms/ios/ios_platform.py

@@ -215,7 +215,7 @@ def runBenchmark(self, cmd, *args, **kwargs):
                 getLogger().info("\n\t[ ======= Benchmark Logs ======= ]")
                 getLogger().info(logfile_contents)
                 getLogger().info("\t[ ===== End Benchmark Logs ===== ]\n")
-                log_screen += logfile_contents
+                log_screen = logfile_contents
                 logfile_reader.close()
 
         return log_screen, meta