[res] Add a sorted_vector_set container

std::flat_set<> isn't implemented in libc++ yet, and it's one
of the most efficient set implementations. Let's have it under
a slightly different name, as the interface isn't exactly the
same.

Test: atest libandroidfw_tests
Flag: EXEMPT purely new container with no usages
Change-Id: Iddcae8b1098507dbd6c8087c506a8cc4cfecaa57