Allow appdomains to write on cgroup so it can start threads.

Addresses the following denial:

  avc: denied { write } for path="/dev/cpuctl/bg_non_interactive/tasks" dev="cgroup" ino=716 scontext=u:r:shell:s0 tcontext=u:object_r:cgroup:s0 tclass=file permissive=0

which started occurring because of https://android-review.googlesource.com/184260

Bug: 25945485
Change-Id: I6dcfb4bcfc473478e01e0e4690abf84c24128045