Exclude the ActivityStack when processing the reset of a task

When processing the reset of task by ResetTargetTaskHelper, we should
prevent to process task with ActivityStack case. This CL exclude the
root task(ActivityStack) when execute the processing the reset of a task.

Bug: 148590003
Test: atest WmTests:ActivityStackTests
      atest WmTests:TaskTests
      atest WmTests:ActivityRecordTests

Change-Id: I63ed4c2d87f7844fc05c6f61434a0923f2ed3d21