TFLite: Accommodate change in std::span size type

The WIP version of std::span in external/libcxx uses a ptrdiff_t size,
but the finalized C++20 std::span uses a size_t size instead. Comparing
the size_t size() against a signed integer causes a
-Werror,-Wsign-compare build error. Cast the size() expression so that
the code compiles with both the old and the new libc++.

Bug: 175635923
Test: m MODULES-IN-frameworks-native
Change-Id: I50cc5354b6205b62918157b69df6cf1aa7bad6cc