Android: Add comment for android_deps cipd.yaml

Added comment to BuildConfigGenerator.groovy and ran:
rm -rf third_party/android_deps/libs/[!O]*
tools/android/roll/android_deps/fetch_all.py --update-all

Bug: None
Change-Id: Id926c0f53585217bbeec40c9deb64f9505564524
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1904617
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Auto-Submit: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714915}

tools/android/roll/android_deps/buildSrc/src/main/groovy/BuildConfigGenerator.groovy

@@ -459,6 +459,8 @@ class BuildConfigGenerator extends DefaultTask {
 
         // NOTE: the fetch_all.py script relies on the format of this file!
         // See fetch_all.py:GetCipdPackageInfo().
+        // NOTE: keep the copyright year 2018 until this generated code is
+        //       updated, avoiding annual churn of all cipd.yaml files.
         def str = """\
         # Copyright 2018 The Chromium Authors. All rights reserved.
         # Use of this source code is governed by a BSD-style license that can be