[SB][Notif] Add a subtle outline to notification chips.

Now that the chips are system theme color, we need an outline to ensure
the chip is visible on all backgrounds. See b/394611253#comment2 for
screenshots.

Bug: 394611253
Bug: 364653005
Flag: com.android.systemui.status_bar_notification_chips

Test: Trigger notif chip -> verify outline shown around chip (verify
with chips_modernization both OFF and ON)
Test: Trigger notif chip, then dismiss it, then trigger screen share
chip -> verify screen share chip doesn't have an outline

Change-Id: I63ae4d12d438b0e1a88fb8f1dfb896188d43e831