[native] Increase contrast between ColorPickerModal and background view

Summary:
Added a drop shadow to the ColorPickerModal to increase contrast between the modal and the background.

Talked to @atul about this -- but the shadow is noticeably jagged/not smooth around the edge of the modal. You can see the change in shadow color in the circled red spots here:

New to React Native, so I'm open to approaches about how to make this look better!

• (Current approach) Continue using Shadow Props, but fine-tuning the actual values (height, width, or the shadowRadius) to make the shadow smoother
• Not using a drop shadow, but a stroke or something similar instead
• Another styling approach

Test Plan:
Tested on iOS 15.4 and works as expected

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D3846