fs_mgr: 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 libfs_mgr
Change-Id: Ic7f50453f05293b7684be22393d6e5871e493983