Fix iteration through a slice to account for -Wsign-compare

It seems that pi-dev and AOSP compile flags might be different, as
no -Wsign-compare failures happened on AOSP.

Switching from for (int i = 0; ...) style to for (; !slice.empty(); ...)
in several places also provides a better example of how to use Slice.

Test: netdutils_test passes
Bug: 78250686
Change-Id: Ic41d3e77810e41f7ae8573ea1cadbdf4808fa461