[Notif redesign] Align actions to larger icon

Updating the margin of notification actions to fit the new content
margin for the larger "small" icon. We no longer need a separate value
for the conversation margin.

Note that I opted to call this new dimension "margin" instead of
"padding" because although it's called a padding in
NotificationActionListLayout, it's not actually used as a padding.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I6ee981e0677dbfb98bbe15cc68ed9691d4a41f2f