[Refactor] PieChartStatisticViewModel: using by delegation for mutable states

[shamim-emon]

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• I've read the Contribution Guidelines and my PR doesn't break the rules.
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What's changed?

Describe with a few bullets what's new:

• I've updated PieChartStatisticViewModel such that all it's mutableStates uses by delegates

Risk factors

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In what cases won't your code work?

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Does this PR close any GitHub issues? (do not delete)

• Closes [Refactor] PieChartStatisticViewModel: use by delegation for mutable states #3526

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