[Notif redesign] Remove large icon bottom margin

We already use the empty actions_container as bottom padding for the
notification, so the large icon doesn't need its own bottom spacing. If
we kept both, the notification would sometimes end up with too much
space at the bottom.

Bug: 417515478
Test: manual (different font sizes) + screenshot tests
Flag: android.app.notifications_redesign_templates
Change-Id: Iccd8ac5d258c5bbb237212f768b7880525d51538