Fix frozen StretchingOverscrollIndicator animation

[gilnobrega]

StretchingOverscrollIndicator's controller does not have a minimum value for its animation duration.
When the OverscrollNotification's velocity is small enough (< 25) the controller's absorbImpact method sets this animation duration to 0ms, making the animation appear frozen to the user.

This PR sets a minimum animation duration of 50ms.

Fixes #146277

Before	After
Screen.Recording.2024-04-04.at.16.07.43.mov	Screen.Recording.2024-04-04.at.16.34.17.mov

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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[gilnobrega]

@Piinks Google testing step failed again so I thought I'd update the branch in #146280 with the latest master.

I managed to ruin the commit tree so I've opened this PR instead.

Please have a look at the constants introduced here as per the feedback left by @chunhtai

[chunhtai]

for const, you should start with _k

[chunhtai]

LGTM

[Piinks]

Ah sorry about the rebase issue, thanks for sticking with it! Much appreciated contribution, LGTM. :)

The Google testing shard will likely fail again, we'll just need to set it green when it does to land.