With this PR, Slider stops painting the value indicator when activationAnimation is dismissed.

Fixes #180767.

This change has no effect on default value indicator shapes, which animates with activationAnimation and do not paint anything when the animation is dismissed. However, for custom value indicator shapes that do not animate, this allows value indicators to be properly hidden after the animation duration.

For testing, this PR refactors an existing test case, since I consider the new test case a superset of the previous one and improves its capability.

• Previously the test case only verifies the status before dragging and during dragging. Now it also verifies that after dragging.
• Previously the test case uses the default value indicator shapes. Now it uses a custom one that does not animate.

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.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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