Update xTaskGetIdleTaskHandle() For SMP (#868)

Update xTaskGetIdleTaskHandle() For SMP (#868)

* Update xTaskGetIdleTaskHandle() in SMP

This commit updates xTaskGetIdleTaskHandle() for SMP in the following ways:

- xTaskGetIdleTaskHandle() no longer accepts xCoreID argument in SMP so that
there is not change in API between single-core and SMP
- xTaskGetIdleTaskHandle() now returns the Active idle task handle in SMP,
which matches the behavior in single-core.
- Added xTaskGetIdleTaskHandleForCore() in SMP which accepts an xCoreID
argument. This function can be used to obtain the Passive idle task handles.

* Update xTaskGetIdleTaskHandle

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Co-authored-by: Ching-Hsin Lee <chinglee@amazon.com>
Co-authored-by: chinglee-iot <61685396+chinglee-iot@users.noreply.github.com>
Co-authored-by: Rahul Kar <118818625+kar-rahul-aws@users.noreply.github.com>
Co-authored-by: Gaurav-Aggarwal-AWS <33462878+aggarg@users.noreply.github.com>