[Bug]: Text inside NiaBackground has wrong colors in some cases

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Is there a StackOverflow question about this issue?

- [X] I have searched StackOverflow

### What happened?

As seen in https://github.com/android/nowinandroid/issues/917, some NiaBackground screenshots don't show any text or they show it with very low contrast.

Chatted with @alexvanyo about this:
```
I wonder if we need to manually set the contentColor in the Surface - I think the default behavior is it tries to determine it from the background color, but maybe it doesn't work with our custom ones?
```


### Relevant logcat output

_No response_

### Code of Conduct

- [X] I agree to follow this project's Code of Conduct

Locate the NiaBackground composable and reproduce the low-contrast or missing text shown in the linked issue screenshot. Inspect how its Surface background and content color are resolved; done means text inside NiaBackground remains visible with sufficient contrast in the affected cases.

Written by the indexing model from the issue text.