binder: 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
llvm.org/PR96319.

Bug: http://b/349681543
Test: m libbinder_on_trusty_mock
Test: m MODULES-IN-frameworks-native
Change-Id: Ibdd9b30c6e913b0785250b2a034ca3b193355032