kernel: POSIX: correcting default thread prio & policy in attr.
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 <7f33386d52951c3e7a699b9509267436d9be79f8@intel.com>

nashif/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,kraj/zephyr,ldts/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,galak/zephyr,Vudentz/zephyr,mbolivar/zephyr,explora26/zephyr,galak/zephyr,zephyriot/zephyr,ldts/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,Vudentz/zephyr,explora26/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,nashif/zephyr,nashif/zephyr,zephyriot/zephyr,finikorg/zephyr,mbolivar/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,kraj/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,punitvara/zephyr,punitvara/zephyr,galak/zephyr,zephyriot/zephyr,ldts/zephyr,mbolivar/zephyr,finikorg/zephyr,punitvara/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyriot/zephyr,kraj/zephyr,ldts/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,punitvara/zephyr,Vudentz/zephyr