[SPARK-19567][CORE][SCHEDULER] Support some Schedulable variables immutability and access

## What changes were proposed in this pull request?
Some `Schedulable` Entities(`Pool` and `TaskSetManager`) variables need refactoring for _immutability_ and _access modifiers_ levels as follows:
- From `var` to `val` (if there is no requirement): This is important to support immutability as much as possible.
  - Sample => `Pool`: `weight`, `minShare`, `priority`, `name` and `taskSetSchedulingAlgorithm`.
- Access modifiers: Specially, `var`s access needs to be restricted from other parts of codebase to prevent potential side effects.
  - `TaskSetManager`: `tasksSuccessful`, `totalResultSize`, `calculatedTasks` etc...

This PR is related with #15604 and has been created seperatedly to keep patch content as isolated and to help the reviewers.

## How was this patch tested?
Added new UTs and existing UT coverage.

Author: erenavsarogullari <erenavsarogullari@gmail.com>

Closes #16905 from erenavsarogullari/SPARK-19567.