refactor `getAppsByState`, `getRejectedAppsByState`, and `getCompletedAppsByState`

`getAppsByState`, `getRejectedAppsByState`, and `getCompletedAppsByState` do the similar job that get apps according to specific state. Hence, it would be better to have a helper method and then these three methods can reuse the code.

https://github.com/apache/yunikorn-core/blob/master/pkg/scheduler/partition.go#L1075

https://github.com/apache/yunikorn-core/blob/master/pkg/scheduler/partition.go#L1088

https://github.com/apache/yunikorn-core/blob/master/pkg/scheduler/partition.go#L1101