aapt2: 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 libaapt2
Change-Id: Ic712f8c844b05687948162e5d080af3726b322f6