fix(core): __symbol__ should return __zone_symbol__ without zone.js loaded

[JiaLiPassion]

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?

Issue Number: N/A
#19534

What is the new behavior?

will not check isInAngularZone if zone.js not loaded.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

When not loading zone.js, __symbol__ function should still return __zone_symbol__ string for zone.js loaded check.

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