[native] Make ColorSelector fully functional

Summary:
Made ColorSelector fully functional: added onPress handling, save buttons for the thread color, and modified the ColorPickerModal height to fit the smaller size of ColorSelector (as opposed to ColorPicker).

Full context in Linear issue. Phasing out ColorPicker can be handled in a future diff. Depends on D3868.

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

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D3939