Fix thread priorities for unstarted threads.

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