Allow to drop existing fd cache of cgroup path

A process can give up the permission to set cgroup. If we still
keep the fd that was cached before losing the permission, when
the process sets scheduling group, it will write to the cached
fd without checking if is accessible and lead to sepolicy denied.

Bug: 123043091
Test: Build and boot.
Test: A new process from zygote set cgroup and drop fd cache, and
      then specializes to app domain. There is no sepolicy denied
      when the process creates new thread.
      (android::thread_data_t::trampoline)

Change-Id: I285ee91424ea965ea9c670fc0f6662948e3e2ce5