Enlarge preview button touch area to meet GAR criteria

GAR define view of touch area should at lease large than 48dp
We keep original icon size, but enlarge touch area.

Fix: 130207526
Test: manual use a11y scanner to check
Test: atest DocumentsUIGoogleTests
Change-Id: I3e59cd652e7ea7c227cd244f7da864f5e7c86535