fix: Make the search box border consistent in all themes

[DaveTryon]

Details

As called out in #1758, the search box doesn't meet non-text color contrast requirements. This PR changes the border so that it's consistently displayed in all themes.

Motivation

Address #1758

Screenshots

Mode	Before (only dark mode has border)	After (all modes have border)
Light
Dark
Aquatic
Desert
Dusk
Night Sky

Context

Since the SearchPanelBorderThickness value is now always 1, I replaced the variable (originally added for dark mode) with a hardcoded value

Pull request checklist

• Run through of all test scenarios completed?
• Does this address an existing issue? If yes, Issue# - 1758
• Includes UI changes?
  • Run the production version of Accessibility Insights for Windows against a version with changes.
  • Attach any screenshots / GIF's that are applicable.

Note: After the PR has been created, certain checks will be kicked off. All of these checks must pass before a merge.

[peterdur]

Built and verified behavior before and after change with default and HC themes