Add default log_path to AndroidDevice.

Bug=28982956

Change-Id: I2a198d64d26b4d13e0cbf71a69dd885589cb743d
diff --git a/acts/framework/acts/controllers/android_device.py b/acts/framework/acts/controllers/android_device.py
index ee3a647..bff4a17 100644
--- a/acts/framework/acts/controllers/android_device.py
+++ b/acts/framework/acts/controllers/android_device.py
@@ -279,8 +279,9 @@
         self.serial = serial
         self.h_port = host_port
         self.d_port = device_port
-        self.log_path = os.path.join(logging.log_path,
-                                     "AndroidDevice%s" % serial)
+        # logging.log_path only exists when this is used in an ACTS test run.
+        log_path_base = getattr(logging, "log_path", "/tmp/logs")
+        self.log_path = os.path.join(log_path_base, "AndroidDevice%s" % serial)
         self._droid_sessions = {}
         self._event_dispatchers = {}
         self.adb_logcat_process = None