CallStyle: Evenly divide space for action buttons

Add an "evenly divided" mode to NotificationActionListLayout that
divides the width into even shares and stretches each action button to
fill its share.

Use it in Notification.CallStyle to fulfill the new CallStyle action
button layout.

Bug: 268733030
Test: manual; sUseNewActionLayout = true, post CallStyle with Notify2
Change-Id: I5e8483d9dbe7c7faa723c41e69939fa98e1fc0f0