Align ActivityData.feeds to use FeedId instead of strings

[gpunto]

Goal

AND-816

We're using FeedId in models to represent group+id of a feed, while we were using String in ActivityData.feeds. So we're aligning it.

Implementation

Change from List<String> to List<FeedId> and address the resulting errors.

Testing

No behavior changes.

Checklist

• Issue linked (if any)
• Tests/docs updated
• I have signed the Stream CLA (required for external contributors)

[github-actions]

PR checklist ✅

All required conditions are satisfied:

• Title length is OK (or ignored by label).
• At least one pr: label exists.
• Sections ### Goal, ### Implementation, and ### Testing are filled.

🎉 Great job! This PR is ready for review.

[github-actions]

SDK Size Comparison 📏

SDK	Before	After	Difference	Status
stream-feeds-android-client	2.37 MB	2.37 MB	0.00 MB	🟢

[sonarqubecloud]

Quality Gate passed

Issues
5 New issues
0 Accepted issues

Measures
0 Security Hotspots
87.7% Coverage on New Code
0.1% Duplication on New Code

See analysis details on SonarQube Cloud