fix(account-tree-controller): use getSelectedMultichainAccount

[ccharly]

Explanation

Back-porting fix from:

• fix: account tree controller incorrectly syncing selected account on startup for non-evm accounts metamask-mobile#19692

Basically, coming from our old account model, the selected account could be a Solana account, thus, it might exactly match (speaking of group index here) the equivalent EVM getSelectedAccount.

To fix this, we use getSelectedMultichainAccount which can either select a EVM or non-EVM account, thus, allowing the controller to pick the right account group.

References

N/A

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed, highlighting breaking changes as necessary
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes