auto_updater: Moving common AU constants into a new file

This CL is responsible for moving common AU (internal) contants into
a new file auto_updater_transfer.py so that they can be used within that
file. Usage changed in related files as well.

BUG=chromium:1001312
TESTS=
Ran inside chroot: auto_updater_unittest
Ran inside chroot: flash_unittest

TESTS=
Ran inside chroot: `cros flash --debug <IP> local/samus/latest`
Ran inside chroot: `cros flash --debug <IP>
xbuddy://remote/samus/latest`
Ran inside chroot: `cros flash --debug <IP> chromiumos_test_image.bin

TEST= autoupdate_EndToEnd auto tests (instructions in its control file).

Change-Id: I6170bec98686569deb4826df66a8dc0825a0178f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1829966
Tested-by: Sanika Kulkarni <sanikak@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Commit-Queue: Sanika Kulkarni <sanikak@chromium.org>
Commit-Queue: Amin Hassani <ahassani@chromium.org>
Auto-Submit: Sanika Kulkarni <sanikak@chromium.org>
diff --git a/cli/flash.py b/cli/flash.py
index 912c3c2..e2ef956 100644
--- a/cli/flash.py
+++ b/cli/flash.py
@@ -15,6 +15,7 @@
 
 from chromite.lib import constants
 from chromite.lib import auto_updater
+from chromite.lib import auto_updater_transfer
 from chromite.lib import commandline
 from chromite.lib import cros_build_lib
 from chromite.lib import cros_logging as logging
@@ -447,12 +448,13 @@
                    image_path, payload_dir)
 
     # Generate rootfs and stateful update payloads if they do not exist.
-    payload_path = os.path.join(payload_dir, auto_updater.ROOTFS_FILENAME)
+    payload_path = os.path.join(payload_dir,
+                                auto_updater_transfer.ROOTFS_FILENAME)
     if not os.path.exists(payload_path):
       paygen_payload_lib.GenerateUpdatePayload(
           image_path, payload_path, src_image=self.src_image_to_delta)
-    if not os.path.exists(os.path.join(payload_dir,
-                                       auto_updater.STATEFUL_FILENAME)):
+    if not os.path.exists(os.path.join(
+        payload_dir, auto_updater_transfer.STATEFUL_FILENAME)):
       paygen_stateful_payload_lib.GenerateStatefulPayload(image_path,
                                                           payload_dir)
     return payload_dir