fs_mgr: avoid std::vector<const T>

std::vector<const T> uses std::allocator<const T>, which is an
undocumented libc++ extension to the C++ standard library. The extension
was removed in llvm.org/PR96319. Use an ordinary non-const T instead.

Bug: http://b/349681543
Bug: http://b/375478563
Test: m libfs_mgr
Flag: EXEMPT, refactor to fix build failure
Change-Id: I759526c7507824dca0e2e4f5cb2bf3c58dece1d8