[Notif redesign] Update expand button

Update the look of the expand button to match the redesign spec.

Note that:
- notification_2025_expand_button_pill_bg is a fork of
  expand_button_pill_bg
- notification_2025_expand_button is a fork of
  notification_expand_button
- ic_notification_2025_expand and ic_notification_2025_collapse
  correspond to keyboard_arrow_down_24px and keyboard_arrow_up_24px
  from the Google Symbols library, unchanged.

Bug: 378660052
Test: visual test, screenshot tests to come later
Flag: android.app.notifications_redesign_templates
Change-Id: I91f7e3beef9874b863f6d66b70622e88830c45ef