Make "/*@noflip*/ /*@embed*/" annotation work without CSSJanus hacks

Make "/*@noflip*/ /*@embed*/" annotation work without CSSJanus hacks

This reverts most of commit 2d842f14250646475b5c2ffa2fe4f5a131f94236,
leaving only the test added in it, and reimplements the same
functionality better.

Instead of stripping /*@noflip*/ annotations in CSSJanus, which is
incompatible with other implementations that preserve it, extend
CSSMin to allow other CSS comments to be present before the
rule-global @embed annotation. (This required making the regex logic
in it even worse than it was, but it's actually slightly less terrible
than I expected it would be. Good thing we have tests!)

Bug: 69698
Change-Id: I58603ef64f7d7cdc6461b34721a4d6b15f15ad79