Scheduler: Same priority Preemptive threads should get equal time slice

If there are multiple preemptive threads with same priority, and any
one thread preempts before its time slice expires (due to yields/
semaphore take/queue etc), then next schedules thread is getting
lower time slide than expected.
This patch fixes this issue by accounting time expired when a thread
releases CPU before its time slide expires.

Jira: ZEP-2217/ZEP-2218

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