utils/android: Don't lock up if LogcatMonitor stopped before start

Calling stop before start will result in hanging in self._started.wait(),
because that event will never get set. Although stop before start is an illegal
usage pattern, let's try not to fail annoyingly.

devlib/utils/android.py

@@ -589,6 +589,10 @@ class LogcatMonitor(threading.Thread):
                 self._add_line(line)
 
     def stop(self):
+        if not self.is_alive():
+            logger.warning('LogcatMonitor.stop called before start')
+            return
+
         # Make sure we've started before we try to kill anything
         self._started.wait()