bootstat: avoid vector<const T>

A container of const T uses std::allocator<const T>, which was an
undocumented libc++ extension that has been removed.

See https://github.com/llvm/llvm-project/pull/96319.

Bug: 349681543
Test: m bootstat
Change-Id: Id338f439aa4caf8c9f3c6fc15faef19b1edc4368