[iOS] Fix shell search handler background color for iOS 13 or newer

[dennisstromberg]

Description of Change

Change property to set background color in iOS13 or newer.

Issues Resolved

• fixes Shell.SearchHandler background property not working on iOS[Bug]  #13774
• fixes [Bug] Xamarin.Forms Shell.SearchHandler BackgroundColor not applied in iOS 13 #9887

Platforms Affected

• iOS

Behavioral/Visual Changes

Search handler will show the right background color.

Before/After Screenshots

Before:

After:

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)

[dnfadmin]

All CLA requirements met.

[PureWeen]

/azp run

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Azure Pipelines successfully started running 1 pipeline(s).

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[jfversluis]

Sorry it took a while @dennisstromberg but this is great, thank you so much for your PR!

[jfversluis]

Look forward to the next release notes where you will get an honorable mention as a new contributor! Thanks again @dennisstromberg !

[jsauve]

I'm very much looking forward to this fix. What version of Xamarin Forms will it be included in? I'd implement it myself today in a custom renderer, but it's all contained within a private method, and uses a bunch of internal extension methods.

[jfversluis]

Great @jsauve ! It should be in the upcoming Service Release which is planned for around December 15th. You can already try our nightly (see instructions on the README) which should have all the fixes that will lead up to the new service release. That way you can test if it works as intended. Please be sure to let me know :)