libstagefright_bufferpool: avoid list<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_bufferpool@2.0.1
Change-Id: If562374c45fa5a48b1cf45007b6264937014dea5