SECURITY: Make SVG @import checking case insensitive

@import in embedded CSS is case-insensitive, meaning
an attacker can put "@iMpOrT" and it should still
work.

This uses stripos instead of strpos to make the check
case insensitive.

Bug: T85349
Change-Id: I31db9d81f46460af2d8d3f161ba46c2ab7a170d1