[Notif redesign] Expander: use paddings, not margins

This improves the touch area when the button is centered and when the
notification is expanded - it ensures that the touch target is always at
least the size of the button + 16dp in all directions (so at least
60x52, which is larger than the recommended 48x48 for a11y).

Bug: 378660052
Test: used the hierarchy viewer to check the bounds of the Button
Flag: android.app.notifications_redesign_templates
Change-Id: I7eebe058588a398b56362fd8b66028a76fb9a267