Update Backlinks checkbox to match mocks

- The checkbox wasn't visible in darkmode when unchecked. Setting a tint
  also wasn't helping. So updated to use a custom circular one with the
  right colours.
- For some reason the older text color didn't behave correctly with dark
  mode. So, updated to use a similar variant but of material colour.
- Updated the shape of the checkbox to circular.

Bug: 300307759
Change-Id: I550473267b76c8af9220284825a8b3d53edd4358
Test: Manually verified the text colour for light and dark themes
Flag: com.android.systemui.app_clips_backlinks