PageHeader DropDownButton Flyout Fix

[karkarl]

Description

• Tooltip for info button under "Source" was not accessible. Updated the fontIcon to be in a button for better accessibility.
• Updated flyout children layout to be aligned with Fluent styling.

Motivation and Context

Fixes internal but: Bug 52525763: [WinUI3 Gallery]: Keyboard navigation: Unable to access 'Info' button next to 'Sample Page Source Code' in 'Source' Pop-Up.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[karkarl]

/azp run