CallStyle: Center icon with label on action buttons

When NotificationActionListLayout stretches action buttons to fit their
share of the available width, TextView centers the *label* but leaves
the *icon* at the start edge of the button.

We'd like the icon and the label to be centered together, so add a
"glue" method that will include the icon in the label as an ImageSpan.

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: Idb3fdff9a968cd9d95baafc6ca43f3aacba84d24