Fix UIController not keeping the player at the video's aspect ratio

[MattiasBuelens]

Summary

• Modifier.constrainedAspectRatio has had no effect since v1.4.0 (ef51a33, "Optimize Modifier.constrainedAspectRatio"). ConstrainedAspectRatioNode implemented the legacy LayoutModifier interface instead of LayoutModifierNode, and Compose only registers a Modifier.Node as a layout node if it implements LayoutModifierNode (see calculateNodeKindSetFrom(node: Modifier.Node)). So measure() was never called and the modifier passed sizes through unchanged.
• Effect: UIController and DefaultUI (mobile) didn't keep the player at the video's aspect ratio, or at 16:9 before the video size is known. They filled whatever space the parent gave them. The TV UI doesn't use this modifier.
• Fix: implement LayoutModifierNode. The measure and intrinsic overrides have the same signatures, so nothing else changes.
• This also clears the two lint warnings on ConstrainedAspectRatioElement.create() (ModifierFactoryExtensionFunction, ModifierFactoryReturnType). Lint was treating create() as a modifier factory because the node was a Modifier type through LayoutModifier.

Test plan

• New ConstrainedAspectRatioTest (6 tests): height derived from a fixed width, from a square ratio and from fillMaxWidth(); width derived from a fixed height; incoming constraints are never violated; a real UIController in a 320 × 1000 dp box measures 320 × 180 dp
• 5 of the 6 tests fail without the fix (for example, UIController measures 320 × 1000 dp). The constraints test passes either way, because the fixed size passes straight through.
• ./gradlew :ui:testDebugUnitTest
• ./gradlew :ui:lintDebug: no issues found

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