Use a fixed size for smart actions icons.

Before this CL we would calculate the size of smart action icons by
inflating a new button (to make sure the icon wouldn't make the button
larger than necessary). With this CL we instead use a fixed size for the
icon.

Bug: 119858891
Test: Turn on Smart Actions in cinek@'s Notify app to ensure the icons
have the same size as before this CL.

Change-Id: I596ae3c314770663d9987f80216f368d1eb0007c