SECURITY: Escape '<' and ']]>' in inline <style> blocks

SECURITY: Escape '<' and ']]>' in inline <style> blocks

This is to prevent people from closing the <style> tag, and
then doing arbitrary js-y things. In particular, this is needed
for when previewing user css pages.

This does not escape '>' since its used as the child selector
in css, and generally speaking, '>' is safe inside the contents
of elements.

Bug: T133147
Change-Id: If024398d7bd4b578ad7f8c74367787f5b19eb9d7