Fix some usage of removed private Skia functions & transitive dep

Attempting to fix broken roll.

sk_float_* functions map directly to <cmath> functions now, see
https://skia-review.googlesource.com/c/skia/+/842498

SkiaInterpolator requires <cstring> for memcpy. It seems this was
previously being pulled in through a transitive Skia inclusion which
is no longer present.

Bug: none
Change-Id: I155ead6c68eab066fb4d5588f35523aff2c4a050
Test: builds
Flag: EXEMPT build fix