Add a log when a device's debug tag changes. (#196)

This helps correlate the old and new debug tags.
diff --git a/mobly/controllers/android_device.py b/mobly/controllers/android_device.py
index 159f587..d9fd951 100644
--- a/mobly/controllers/android_device.py
+++ b/mobly/controllers/android_device.py
@@ -350,7 +350,7 @@
                   android device should be stored.
         log: A logger adapted from root logger with an added prefix specific
              to an AndroidDevice instance. The default prefix is
-             [AndroidDevice|<serial>]. Use self.set_debug_tag to use a
+             [AndroidDevice|<serial>]. Use self.debug_tag = 'tag' to use a
              different tag in the prefix.
         adb_logcat_file_path: A string that's the full path to the adb logcat
                               file collected, if any.
@@ -408,6 +408,7 @@
             The tag can be customized with `ad.debug_tag = 'Caller'`:
                 'INFO [AndroidDevice|Caller] One pending call ringing.'
         """
+        self.log.info('Logging debug tag set to "%s"', tag)
         self._debug_tag = tag
         self.log.extra['tag'] = tag