libsnapshot: replace non-character basic_string[_view]<T>

In newer versions of libc++, std::char_traits<T> is no longer defined
for non-character types, and a result, std::basic_string<T> and
std::basic_string_view<T> are also no longer defined for non-character
types. See
https://discourse.llvm.org/t/deprecating-std-string-t-for-non-character-t/66779.

Replace them with std::vector<T> and std::span<const T>.

Bug: 175635923
Test: m MODULES-IN-system-core-fs_mgr
Test: /data/nativetest64/cow_api_test/cow_api_test
Change-Id: Ife2e87833ced43ff24e5765998cb6993e4f9b4c0