Grit: Preserve license headers in built-in minification on Android.

Based on past discussions, license headers should be preserved in
distributed JS code, even when minification is present.

This is done as a precursor step to fully removing Grit's built-in
minification, as an illustration that Grit's built-in minification is no
longer necessary since it mostly operates on already minified files
produced by other parts of the build leveraging minify_js() from
ui/webui/resources/tools/minify_js.py.

Specifically, this will help get more accurate results from the
android-binary-size bot, when Grit's minification is removed in a
follow-up CL. The reported size diff is currently inflated because of
Grit erroneously removing license headers when operating on already
minified files.

Bug: 340278433
Change-Id: I0f8d75ffa797db58292b04bdefbd86e46bae2932
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6318882
Auto-Submit: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Andrew Grieve <agrieve@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1427505}