Disposing the unavailable images evicted by ImageCache in time

[0xZOne]

Avoid additional keepalive to prevent disposing unavailable images that have already been evicted by ImageCache.

Fixes https://github.com/flutter/flutter/issues/125608

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[0xZOne]

This will not actually release memory.

In the case under test, the image render object is still in the tree holding onto the ui.Image.

Actually, the image has already been evicted by ImageCache and is no longer available. We should dispose of it in time to release memory as soon as possible.

[goderbauer]

(Triage) @0xZOne @dnfield Are there still plans for this PR?

[goderbauer]

(Triage) Since there was no follow-up I am going to close this one. Feel free to reopen if you find the time to work on this again. Thank you for your contribution!