Fix workaround in Segmented control

[AndrewKeepCoding]

This commit modifies the OnApplyTemplate method in the Segmented class to set SelectedIndex to -1 and then assign _internalSelectedIndex if _hasLoaded is false. This ensures the selected index is reset before applying the internal selection and marks the template as loaded.

Fixes

#698

PR Type

What kind of change does this PR introduce?

• Bugfix

What is the current behavior?

The selected item is not rendered as selected when in XAML:

• 2 SegmentedItems are declared
  and
• SelectedIndex is set to 1.

<toolkit:Segmented SelectedIndex="1">
    <toolkit:SegmentedItem Content="Item 1" />
    <toolkit:SegmentedItem Content="Item 2" />
</toolkit:Segmented>

What is the new behavior?

The selected item is rendered as expected.

PR Checklist

Please check if your PR fulfills the following requirements:

• Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• Based off latest main branch of toolkit
• Contains NO breaking changes

Other information

[Arlodotexe]

I was able to reproduce the issue locally and have confirmed that this change fixes it. Thanks, approved!