Picker Attribute "SelectedIndex" Not being respected on page load - fix

[kubaflo]

Fixes #9150

For the following code:

        <Picker FontSize="15"
                AutomationId="picker"
                SelectedIndex="1"
                x:Name="picker">
            <Picker.ItemsSource>
                <x:Array Type="{x:Type x:String}">
                    <x:String>0</x:String>
                    <x:String>1</x:String>
                    <x:String>2</x:String>
                </x:Array>
            </Picker.ItemsSource>
        </Picker>

Before	After

[jsuarezruiz]

/azp run

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Azure Pipelines successfully started running 3 pipeline(s).

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[jsuarezruiz]

@kubaflo Could you rebase to fix the conflicts? Thanks in advance.

[kubaflo]

@jsuarezruiz I've rebased and solved conflicts :)

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).