Fix keyboard_hot_restart_ios flakes

[loic-sharma]

The keyboard hot restart test does these steps:

1. It launches an app that forces the keyboard to be visible
2. It checks that the keyboard is visible by examining the app's logs
3. It updates the app to no longer force the keyboard to be visible and hot restarts the app
4. It checks that the keyboard is no longer visible by examining the app's logs

Previously, the test app printed the keyboard status whenever the app rebuilt. The test would move on as soon as it saw a log that indicates the keyboard was in the expected state. However, since the app could log the keyboard's visibility before the Dart VM was available, hot restart commands could be ignored (or at least, that's my theory from the DeviceLab logs).

Now, the test app prints the keyboard status non-stop. The test now waits until the Dart VM is available before checking if the logs indicate the keyboard is open. Also, the test now waits until the hot restart has completed before checking if the keyboard is closed. In theory, this should ensure that hot restart does not happen until after the Dart VM is available.

Follow-up to: #167013
Part of: #10713

Pre-launch Checklist

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• 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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[justinmc]

LGTM 👍 . Sorry I missed this notification, thanks for pinging me. Hopefully this stops the flakiness.

[justinmc]

Do you have any idea how long this typically has to wait?

[loic-sharma]

On my machine, this usually takes 1-2 for each of the checks: initial open keyboard check and the final closed keyboard check