Expose the duration and curve for theme animation in MaterialApp.

[darrenaustin]

When a MaterialApp's theme changes, it is animated over time (via the AnimatedTheme widget). This PR exposes the duration and curve used for this animation:

MaterialApp(
  theme: ThemeData.light(),
  darkTheme: ThemeData.dark(),
  themeMode: ThemeMode.dark,
  themeAnimationDuration: const Duration(milliseconds: 750),
  themeAnimationCurve: Curves.bounceInOut,
  // ...

Or if you want to turn off the animation altogether, you can set the duration to zero:

MaterialApp(
  theme: ThemeData.light(),
  darkTheme: ThemeData.dark(),
  themeMode: ThemeMode.dark,
  themeAnimationDuration: Duration.zero,
  // ...

Fixed: #105883

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

[HansMuller]

LGTM

What happens if these parameters are changed while a theme animation is underway?

[goderbauer]

LGTM

[darrenaustin]

What happens if these parameters are changed while a theme animation is underway?

In the underlying AnimatedTheme they will just replace the existing animation with a new one based on the changed curve and set the controller's duration to the new value:

flutter/packages/flutter/lib/src/widgets/implicit_animations.dart

Lines 382 to 389 in f980d66

	@override
	void didUpdateWidget(T oldWidget) {
	super.didUpdateWidget(oldWidget);
	if (widget.curve != oldWidget.curve) {
	(_animation as CurvedAnimation).dispose();
	_animation = _createCurve();
	}
	_controller.duration = widget.duration;

Do you think we need to call this out in the docs for MaterialApp?

[HansMuller]

#107383 (comment) sounds OK, I don't think we need to document that case.