Clamp action bar button height to default minimum height

Previously, the fixed size of 56dp would bump the landscape action bar
measured height up to 56dp but leave the minHeight at 48dp, which would
cause the buttons to be aligned incorrectly. Now the buttons are scaled
to fit the action bar.

Change-Id: I47801d1fb072d8801b7bbae5c42be3bdfb819404