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

(cherry picked from commit 1bd326a5e2aaff06a5bcae9cb2c42a4e8de31401)

Change-Id: Iad362f7c5c8697c349f2b6d7fcba69a4e141883e
diff --git a/test/051-thread/expected.txt b/test/051-thread/expected.txt
index 943d1df..54e34af 100644
--- a/test/051-thread/expected.txt
+++ b/test/051-thread/expected.txt
@@ -9,4 +9,6 @@
 testSetName starting
 testSetName running
 testSetName finished
+testThreadPriorities starting
+testThreadPriorities finished
 thread test done