input: handle change in std::span::size type

The pre-standardized version of std::span in external/libcxx had a
ptrdiff_t size, but the finalized std::span has a size_t size instead.
Also, the std::span::index_type typedef is renamed to size_type. Use an
old-style constructor call to implicitly coerce the size value to the
proper type.

Insert a cast to avoid a signedness comparison warning.

Bug: b/175635923
Test: treehugger
Change-Id: I96ccf6d5b54d4118b096f97c901073b4fc2f6f9f