Remove the padding around checkboxes

[mattleibow]

Description of Change

Since platform checkboxes are designed with the check box and the label as a single control, most have a bit of padding between them.

This PR removes the padding and unifies the size behaviour to just be the check box and some margin..

<VerticalStackLayout Spacing="20" Padding="20">
    <HorizontalStackLayout HorizontalOptions="Start" Background="Silver">
        <CheckBox />
        <Label Text="Hello World" VerticalOptions="Center" />
    </HorizontalStackLayout>
</VerticalStackLayout>

Platform	Before	After
Android
Windows
iOS

NOTE: The Android and Windows checkboxes are square-ish and have a view bunds of about 32px in size. iOS and Mac Catalyst are round and have a view size of 44px (Apple recommended values) - but a circle size of 18px - which results in a bit more padding. However, this is the values we always had so I am leaving it, but the size can be reduced to 32px and the circle will stay centered.

Issues Fixed

Fixes #7352

[jsuarezruiz]

/azp run

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