UnitControl: avoid calling onChange callback twice when unit changes

[ciampo]

What?

Prior to this PR, the onChange callback on UnitControl is called twice when the unit changes.

Why?

Just an error in the component's logic.

This bug was uncovered by the work done in #40790 (comment) while a solution was, at the same time, already being proposed by @stokesman as part of #40568.

This PR extracts the fix from #40568, while also updating the corresponding unit test.

How?

Removed the call to onChange from within the mayUpdateUnit function.

Testing Instructions

• Updated unit tests pass
• Pass both the onChange and onUnitChange callbacks to UnitControl. Interact with the component by changing the unit from the dropdown. Make sure that the both callbacks are called, but that onChange is called only once.

[stokesman]

The changes make sense. Thanks for breaking this out as its own PR 🙌.

Prior to this PR, the onChange callback on UnitControl is called twice when the unit changes.

It's only called twice for unit changes triggered by typing into the text input. Or at least that's the only case this change pertains to but I think it's a detail that's not too important.