[FR]: NiaApp background composable could be improved

### Is there an existing issue for this?

- [X] I have searched the existing issues

### Describe the problem

Context: https://github.com/android/nowinandroid/pull/330/files/ee4eb25ba07e7a017c52f54fa49547a0e733ff8b#r1001237941

From @manuelvicnt:
What's this complexity? Could this be a slot API instead? And the content inside it could be using moveableContent so that state isn't lost

### Describe the solution

Let's discuss this in our weekly.

### Additional context

_No response_

### Code of Conduct

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

Start with the linked pull-request discussion and inspect the NiaApp background composable referenced there. The issue only asks for a weekly discussion about a possible slot API and movable content, so the intended design, files to change, tests, and definition of done are not yet specified.

Written by the indexing model from the issue text.