Remove assert from Image._handleImageFrame() - #33602

Description

Tickers being disabled and re-enabled can cause the
condition of a synchronous notification happening after
image frames have been delivered, which is valid in that
case. As such, this removes the assert.

Related Issues

#32374

Tests

I added the following tests:

• A test of the condition that was incorrectly hitting the assert

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Does your PR require Flutter developers to manually update their apps to accommodate your change?

• Yes, this is a breaking change (Please read [Handling breaking changes]). Replace this with a link to the e-mail where you asked for input on this proposed change.
• No, this is not a breaking change.