CmdPal: Prevent some SearchText bouncing. (#41165)

This stops us from raising a PropChanged(SearchText) in DynamicListPage
when we're the ones to set it.

When we'd raise the PropChanged in response to a `set`, it could cause a
race between CmdPal and the extension. It was totally possible that
CmdPal could call

```
SearchText="foo";
SearchText="fool";
```

and in the extension, we'd raise the PropChanged for each of those, but
then have CmdPal handle those events out-of-order.

This seems to entirely remove all the "jiggling" that I'd notice in the
evil samples from #41158

Closes #38190