Use sp for screenshot button icon size

The icons behave more like text, and can be mixed with text, so
use sp to align the button sizes regardless of whether or not each
button has text.

Bug: 191147490
Fix: 191147490
Test: manual, visual verification (on e.g. Settings to get the
Capture More button)

Change-Id: I813fd5761dc8b3c42f150a61555514c22e33dacc