[native] Add active state to ColorSelector

Summary:
Part of ongoing redesign of ColorPicker component on native to match the ColorSelector component on web. Depends on D3798.

Adds active state to ColorSelector by adding styling and a checkmark to the ColorSelectorButton that is currently selected as the thread color.

Will add onPress handling and updating the thread color in a later diff, as outlined in the plan on this Linear issue.

Test Plan:
Tested on iOS 15.4 and works as expected:

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3868