fix(router): always stringify matrix parameters

[AlexMokin]

Fix a case where matrix parameters weren't stringified when they are passed as a first command
when creating a url tree. Fix return type in parseMatrixParams method
because it always returns {[key: string]: string}

Closes #23165

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Matrix parameters are not stringified when then they are passed as the first command.

Issue Number: #23165

What is the new behavior?

Matrix parameters are always stringified

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[atscott]

presubmit
global presubmit

[atscott]

merge assistance: pullapprove seems to be stuck. Also, this can be merged after the 11.1 release today.

[AndrewKushnir]

FYI, just did a rebase to rerun CI with the most up-to-date version of the codebase, PullApprove got updated as well.

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