Stop using NotificationsHunSharedAnimationValues.assertInNewMode()

Avoid risking crashes in nextfood by replacing the assertInNewMode with
isUnexpectedlyInLegacy mode. If the code path is accidentally used
again, we would break the HUN intro/outro animation, but not crash the
device.

Bug: 400416268
Flag: com.android.systemui.notifications_hun_shared_animation_values
Test: turn off the flag, run HUN intro/outro
Change-Id: I41ad7a0c5972ee670ea7309d0d66508194d1cd3f