Create a new safe region letterboxing trunk stable flag in aaos_core_contrib namespace

- While moving namespaces for the flag previously, the state in gantry got messed up.
- Instead, remove the old flag and add a new one.

Bug: 425949809
Flag: EXEMPT (flag addition)
Test: m
Change-Id: I330118af075aea2afc52ece8aed3321b99a9f58a