* Update to the latest buildifier release.
For the previous buildifier version, the exit code would always be 0
(regardless of whether files needed formatting or not).
Manually confirmed that with version 4.2.3 the test_code_style.sh script
correctly errors out when a BUILD file has formatting errors (and that
it doesn't with the previous version).
Also,
* Updated the docker container and pointing to the new container.
* Fix the formatting errors in .bzl file that crept in because this check was not working in OSS.
BUG=http://b/203803273
* Exclude the downloads directory.
* Updated docs based on review comments.
With this change:
* we are set up to use yapf as part of the docker container
* pigweed patch is updated to use the google Python style (pep8 with indent of 2 spaces)
* some documentation updates.
The code still needs to be formatted, and then the Python formatting check will be turned on as part of ci.
BUG=http://b/194404216
