Update the copyright headers in the Proguard flags files

The Proguard flags files are self-referencing, so update their
copyright headers in sync.

(There are only five files in /third_party/androidx so update the
other three files as well while we're here.)

Bug: 1098010
Change-Id: Ia014b5d559cdfc916117c2f70fae7269471740a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3881211
Owners-Override: Avi Drissman <avi@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Auto-Submit: Avi Drissman <avi@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1044569}