libfilament (including libutils and libmath) are 100% std::string
free.
std::string is pulled in the .so (on android) through libc++ for
exception handling, even if we're not using them. There is not much
we can do here, but at least, it's not because of us!
utils::ostream still references it but only as an inline function,
so if the inline is not called, std::string won't be pulled in.
It's also referenced from Path.cpp, but that's not included in
libfilament.
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