Fix calls to pthread_setschedparam.

When the scheduling priority is set to SCHED_NORMAL, you may not
specify a non-zero priority. The current implementation does
exactly that resulting in the call failing and returning EINVAL.
This change ensures that a priority of 0 is set when the desired
scheduler policy is set to SCHED_NORMAL.

http://b/12304174

Change-Id: I945cae175b7bdd29dd8fc130ff6396b88ec33fcb