Use maintain-resolution as the default video degradation preference (#1106)

## Summary
- Align Flutter default local-video degradation behavior with the Swift
SDK.
- Default unset `VideoPublishOptions.degradationPreference` to
`maintainResolution` for camera and screen-share publishing.
- Keep explicit degradation preferences overrideable by apps.

## Context
Related to #1097, which explores preserving video quality through a
live-streaming option. This PR takes the smaller SDK-default approach
instead: use maintain-resolution by default, matching Swift, without
adding a separate app-facing toggle for this behavior.

## Testing
- `dart analyze`
- `flutter test test/core/room_e2e_test.dart`