Replace std::basic_string<uint8_t> with std::vector

In newer versions of libc++, std::char_traits<T> is no longer defined
for non-character types, and a result, std::basic_string<uint8_t> is
also no longer defined. See
https://discourse.llvm.org/t/deprecating-std-string-t-for-non-character-t/66779.

Bug: 175635923
Test: make checkbuild
Change-Id: Icb3937d8b1ff6dbe7e35e62f2e6cc1e2eb789121