Add background color to selected device and app

[snappdevelopment]

This is a small visual improvement to quickly see which device and app is currently selected when looking at the dropdown menu. When running multiple emulators or app variants it is sometimes hard to see which one is the one that is already selected, so I added a background color to make the selected one stand out a bit more.

[gemini-code-assist]

Code Review

This pull request introduces changes to highlight selected items in both the application and device dropdown menus by applying a background color when selected. The review feedback points out that using .run for conditional modifiers is not idiomatic in Jetpack Compose and suggests using Modifier.then() instead. Additionally, it highlights a semantic color mismatch and contrast issue where onTertiary (a content color) is used as a background color, recommending the use of primary instead to match the hardcoded onPrimary text color.

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