libcompositionengine: 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 libcompositionengine
Change-Id: I359350a298fc9a59c0ca925a36f753ac3fb3b64e