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>

Conflicts:
	src/com/android/incallui/CallButtonFragment.java