bug(ios): system keyboard sometimes blank after device unlock

From general testing of #11179.

> # Test Results
>
> * **TEST_GENERAL_USE (FAILED):** Retested with the attached PR build (Keyman 17.0.305-beta-test-11179) on an iPhone 13 Mobile device (iOS 17.4) and here is my observation: 1. Set Keyman as the system wide keyboard. 2. Opened Safari browser. 3. Switched to Keyman Keyboard. 4. Typed a text on the Search bar. 5. Locked the mobile device by pressing Lock key from the mobile. 6. Unlock the mobile. Here, I noticed that the Keyman Keyboard was no longer visible in the Keyboard panel. Seems to be an issue.
>

Interesting that we haven't seen this one until now. There always have been odd interactions with that specific use case - see #2300 - and I can see the host-page getting reloaded / restored after locking and unlocking. It's _temporarily_ blank for me, rather than permanently blank... but I'm also not sitting and waiting between locking & unlocking. If something were to prevent the host-page reload, that could certainly cause the issue. Of course, _getting evidence_ of that is an entirely different matter.

_Originally posted by @jahorton in https://github.com/keymanapp/keyman/issues/11179#issuecomment-2046965050_

It's hard to say that this is a Safari-specific interaction, of course, but #2300 existing does make it a possibility in my mind.