[native] Minor relationship prompt styling changes

Summary: Friend noticed that Accept Friend Request was taking up two lines (https://blob.sh/atul/old.png). Reduced the font size so it fit on one, added icons, and made the colors a little more vibrant. Here's what it looks like after: https://blob.sh/atul/new_accept_reject.png, https://blob.sh/atul/new_block_unblock.png

Test Plan: No changes in functionality, visually looks as expected.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D882