libstagefright: 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 libstagefright
Change-Id: If6dd5edb7a6c83d5e3f37e21fd8fdfe41f1093cc