Suggesting at most three smart actions

Removed Notification.MAX_ACTION_BUTTONS check as we no longer
show smart actions as normal actions. Instead, suggesting at most
3 smart actions.

BUG: 111437455
Test: Manual, observes smart action even the notification always have
     three normal actions.

Change-Id: I9238ef8b74c4f030979cb36cc87eaeff8c777682