ui: use std::span instead of std::basic_string_view<uint8_t>

The string classes are only defined for character types, and
std::basic_string_view<uint8_t> has been removed now. See
https://discourse.llvm.org/t/deprecating-std-string-t-for-non-character-t/66779.

Cast std::span<T>::size() to size_t because the WIP std::span::size()
has a ptrdiff_t return type instead of size_t.

Bug: 175635923
Test: m MODULES-IN-frameworks-native-libs-ui
Change-Id: I156f1032191c74cc7c3f6b0cdb7dc0393e9e906f