Nullability Annotations to Java Classes

[ParaskP7]

This is the parent issue for the Add Nullability Annotations to Java Classes for JP/WPAndroid & FluxC project.

Platform Request: N/A Project Thread: paaHJt-5fa-p2

This project is about adding missing nullability annotations (@Nullable & @NonNull) to as many Java-related classes as possible, including fields, arguments, parameters, and return values.

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### Tasks
- [x] #18924
- [ ] #18905
- [ ] #18911
- [ ] #18917
- [ ] #18912
- [ ] #18913
- [ ] #18915
- [ ] #18918
- [ ] #18914
- [ ] #18916
- [ ] #18919

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### Stretch Goals
- [ ] #19025
- [ ] #19113

[peril-wordpress-mobile[bot]]

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