[Notif redesign] Expander: center numbered layout better

Reduce the end padding when the number is showing, to better account for
the built-in spacing of the expander icon.

Bug: 378660052
Test: visual test with both RTL and LTR languages, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I3ecede3c0c0ec597d7907d081072c8dbff5a1b89