Fix byte string concat error in _HandleAUTH()

This fixes https://console.cloud.google.com/errors/CMebxsbZ1PzK8gE?time=PT1H&refresh=auto&project=chromium-swarm

Bug: 1198820
Change-Id: I88498d61a544b1107f0c7a138aae0900e4488e4a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/python-adb/+/2943853
Auto-Submit: Junji Watanabe <jwata@google.com>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
diff --git a/adb/adb_protocol.py b/adb/adb_protocol.py
index 7fd2552..684f151 100644
--- a/adb/adb_protocol.py
+++ b/adb/adb_protocol.py
@@ -579,7 +579,7 @@
       # user.
       msg = _AdbMessage.Make(
           b'AUTH', _AdbMessageHeader.AUTH_RSAPUBLICKEY, 0,
-          self._rsa_keys[0].GetPublicKey() + '\0')
+          self._rsa_keys[0].GetPublicKey() + b'\0')
       msg.Write(self._usb)
       try:
         reply = _AdbMessage.Read(self._usb, self._auth_timeout_ms)