Adds tests for `relative_positioned_transition`, `positioned_transition`, `sliver_fade_transition`, `align_transition`, `animated_builder`, `rotation_transition`, `animated_widget`, `slide_transition`, `listenable_builder`, `scale_transition`, `default_text_style_transition`, `decorated_box_transition`, `size_transition` api examples. Makes double type in the `align_transition` example explicit.
A test for `fade_transition` is already in currently open #148178.
Part of #130459.
This PR contributes to https://github.com/flutter/flutter/issues/130459
### Description
- Adds disposal of the `CurvedAnimation` in `examples/api/test/widgets/transitions/fade_transition.0.dart`
- Adds tests for `examples/api/lib/widgets/transitions/fade_transition.0.dart`
## Description
This fixes an issue in the creation of the `FocusScope` in a route: the route should be creating the `FocusScope` widget it has with `withExternalFocusNode`, since it is modifying the node attributes in a builder.
Also modified some `AnimatedBuilder`s to be `ListenableBuilder`s, since they're not using animations (no functionality change there, since the implementation of the two is identical).
## Related Issues
- #147256
- Fixes#146844
## Tests
- Updated example test.
