Fix thread priorities for unstarted threads.

Calls to Thread.setPriority for unstarted threads now behave
similar to dalvik. Note that there's still some inconsistent
behaviour carried over from dalvik.

- high priority threads from bg_non_interactive processes are
  not always moved to the SP_FOREGROUND cgroup.
- we do not attempt to adjust the cgroup of a native thread
  that's attaching.

Note that on android, the system_server will change the
cgroups for all running threads in a process when it moves
into the foreground and background. It's by design that
threads in a background process can request to be moved
to the foreground by setting a higher priority.

bug: 17893086

Change-Id: I1662982b1c7b3ac509698e2e12c9768d082c8053