[iOS] Setting background color on the Searchbar - fix

[kubaflo]

Issues Fixed

Fixes #23325
Fixes #24100

[dotnet-policy-service]

Hey there @kubaflo! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[Redth]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

[mattleibow]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[PureWeen]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

[aheubusch]

@kubaflo @PureWeen I think I see another bug. Once a custom BackgroundColor has been set its not possible to return to the default color (searchHandler.BackgroundColor = null;). Infact it will cause a NRE in line 168 (backGroundColor.ToPlatform()).
Proposed change to line 154

if (backGroundColor == null)
{
	_hasCustomBackground = false;
	
	backgroundView.Layer.CornerRadius = 0;
	backgroundView.ClipsToBounds = false;
	backgroundView.BackgroundColor = _defaultBackgroundColor;
	textField.BackgroundColor = _defaultBackgroundColor;
	
	return;
}

[jsuarezruiz]

/rebase

[jsuarezruiz]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).