Make custom view stripping behaviour consistent between NMS and SysUI

Right now SysUI cancels the oversized notification while NMS sends a
standard temoplate anyway. Losing notifications is bad, so this makes
SysUI behaviour consistent with NMS and repost oversized custom view
notifications as standard template ones.

Bug: 270553691
Flag:com.android.server.notification.notification_custom_view_uri_restriction
Test: manual test with test app + newly added unit tests

Change-Id: I0ab7d668c982b34331bf6836592021f1b0be0bef