kernel: POSIX: correcting default thread prio & policy in attr.

This patch does following:-
1. Default scheduling policy should be set to SCHED_RR only when
Preemptive is enabled.
2. Default priority in attr object should equivalent to
K_LOWEST_APPLICATION_THREAD_PRIO. Posix priority corresponding
to K_LOWEST_APPLICATION_THREAD_PRIO is 1.

Signed-off-by: Youvedeep Singh <youvedeep.singh@intel.com>