fix: Update current UI instance upon locale change

[mshabarov]

Description

Sets the UI on which the locale is being changed as a current UI for the thread. This is needed to have a proper UI.getCurrent() instance inside the executed locale change observers in components.

Fixes #11599

Type of change

• Bugfix
• Feature

Checklist

• I have read the contribution guide: https://vaadin.com/docs/latest/guide/contributing/overview/
• I have added a description following the guideline.
• The issue is created in the corresponding repository and I have referenced it.
• I have added tests to ensure my change is effective and works as intended.
• New and existing tests are passing locally with my change.
• I have performed self-review and corrected misspellings.

Additional for Feature type of change

• Enhancement / new feature was discussed in a corresponding GitHub issue and Acceptance Criteria were created.

[denis-anisimov]

Is it needed ?
Every Flow listener should be called under the session lock arelady.

[mshabarov]

Yeah, it's not needed. VaadinSession::setLocale checks that it already has a lock.

[mshabarov]

Done

[denis-anisimov]

Again: is this needed?

[mshabarov]

No.
Done.

[vaadin-bot]

SonarQube analysis reported 3 issues

1. VaadinSession.java#L201: This block of commented-out lines of code should be removed.
2. VaadinSession.java#L1066: Make the "java.util.Map" class "Serializable" or don't write it.
3. VaadinSession.java#L79: Do not forget to remove this deprecated code someday.

[vaadin-bot]

Hi @mshabarov and @mshabarov, when i performed cherry-pick to this commit to 2.7, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick 18dc465
error: could not apply 18dc465... fix: Update current UI instance upon locale change (#11725)
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add ' or 'git rm '
hint: and commit the result with 'git commit'

[vaadin-bot]

Hi @mshabarov and @mshabarov, when i performed cherry-pick to this commit to 2.6, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick 18dc465
error: could not apply 18dc465... fix: Update current UI instance upon locale change (#11725)
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add ' or 'git rm '
hint: and commit the result with 'git commit'