fix(account-tree-controller): fix duplicate names for out-of-order accounts

[ccharly]

Explanation

We were not re-using the persisted group name when checking for the next available name during group metadata updates.

Meaning that if somehow, we were receiving a :accountAdded before init was called, then the groups metadata would be empty, leading to some duplicate names.

I spotted this issue with mobile E2E and our fixture system.

References

E2E were failing on this PR, because of this bug:

• chore: bump multichain accounts deps metamask-mobile#20255

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