[compose-richtext] Avoid inactive streaming accent layouts

[pat-oai]

What changed

• Skip preparing the streaming accent overlay layout unless an accent is configured and at least one accent animation is active.
• Make the overlay draw modifier a no-op when there are no active accent animations.

Why

The streaming accent API is optional, but the merged implementation prepared a second text layout for every animated rich text leaf before checking whether the accent feature was active. Existing animate = true callers without streamingTextAccent should not pay that extra layout cost.

Validation

• JAVA_HOME=/opt/homebrew/Cellar/openjdk@21/21.0.10/libexec/openjdk.jdk/Contents/Home ./gradlew :richtext-ui:jvmTest
• git diff --check