Shave another 10us off of hwuitask

pthread_cond_signal does not need the mutex
to be held to signal. This results in the thread
waking up trying to grab the lock, failing, the signaling
thread to wake up, release the lock, immediately get preempted
for the signaled thread which can now proceed.

Release the mutex before signaling to avoid the ping-pong
scheduling issue, which shaves another 10us off of this

Change-Id: Ie6bccca031ba6528f357eae8352b74626a6318c7