Scheduling group cleanup

The C++ API for scheduling groups is going away, so use the C API.

Renumber THREAD_GROUP_* constants to have the same values as SP_*.
They are @hide so this should be OK.

The old THREAD_GROUP_FG_BOOST constant is removed, and replaced by a
(private) THREAD_GROUP_FOREGROUND constant that is just a placeholder
to correspond to SP_FOREGROUND.  It is not usable at this level.

Improved performance of android_os_Process_setProcessGroup:
avoid unnecessary syscall to getpriority for each tid.

Fixed error handling in android_os_Process_setThreadGroup.
It raises an exception if set_sched_policy() returns an error.

Add comments and fix typos in existing comments.

Change-Id: Ib922e92c945985c695ef0addbd2b83b8e82865f8