Improve accent color picking

[micahmo]

Pull Request Description

This PR adds a couple improvements to theme picking.

1. Show three-color swatch in settings page and picker.
   • Note: For the secondary color I actually show primaryContainer and for the tertiary color I show secondaryContainer to reflect what the user is most likely to encounter in the app. See the demo.
2. Keep picker open while selecting accents to show a live preview. Add button to close.

Note that the jagged edges look much better on a physical device.

Issue Being Fixed

Issue Number: #457

Screenshots / Recordings

qemu-system-x86_64_0CSMSr7PBV.mp4

Checklist

• Did you update CHANGELOG.md?
• Did you use localized strings where applicable?
• Did you add semanticLabels where applicable for accessibility?

[hjiangsu]

LGTM!