utils/android: Fix inconsistent logfile read mode

As the exoplayer workload did not specify a pre-existing logfile, it is
created for it by default in LogcatMonitor. This default method opens
the logfile in 'byte' mode rather than the expected 'string' mode.

Regex operations that depend on the logfile for event triggering expect it to
be in 'string' mode, which was not the case.

devlib/utils/android.py

@@ -789,7 +789,7 @@ class LogcatMonitor(object):
         if outfile:
             self._logfile = open(outfile, 'w')
         else:
-            self._logfile = tempfile.NamedTemporaryFile()
+            self._logfile = tempfile.NamedTemporaryFile(mode='w')
 
         self.target.clear_logcat()