libutils: Fix race condition in Thread::requestExitAndWait()

Hold a reference to the thread in requestExitAndWait() so the
condition variable it is waiting on will not be destroyed before
its wait() call returns.

Change-Id: If8b6cf84117203926a4180f43f0224469e92a500
Signed-off-by: Mike Lockwood <lockwood@android.com>