Do not use std::vector in android_namespace_t::is_accessible

Avoid constructing vector and walking all the parents of a soinfo
to check if it is accessible. The most likely scenario that the
very first check returns true.

Bug: http://b/35313368
Test: bionic-unit-tests --gtest_filter=dl*:Dl*
Change-Id: I06c65cf61ed1c30e5e454a169de4c41038863587