incallui: fix callbuttons xxhdpi images

Fix addParticipant and modifyCall images to properly fit in xxhdpi resolutions

Change-Id: I9f1ff0517a86a25660295f1302d0d20e2482fd12
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
(cherry picked from commit b9e308e8e8d4ae26b1ea91f428bb03c1ba7dc773)