Format gen_on_device_proto_descriptors.py

* Reformat to follow PEP8 with YAPF to make it easier to autoformat.
* Some minor modifications to avoid some long lines / indent strings.
  * Includes some whitespace changes to generated code

Change-Id: I92ec8d373ec2257e4d218e89fb960fcf87347c33
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5388814


Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1277101}