Fix third-party license information for a number of projects
Also fix a couple of bugs in the license tool.
- When scanning, ignore directories that don't contain any files, on the
assumption that these are DEPS directories for projects not used by our
- Don't recurse into directories in ADDITIONAL_PATHS, just as we don't for
regular third_party/foo directories.
- License file for third_party/mozc was taken from source header.
- LICENSE.chromium_os was copied from Chromium OS source tree.
This fixes 'licenses.py scan' for an Android checkout.