Create a flag for restart menu for CD

Some UI/UX specs are still under discussion, but they can be
adjusted later. As this feature is needed soon for CD launch, or
at least useful for manual testing, we'd like to start the
development of this feature behind the flag now.

Flag: EXEMPT adding flag
Bug: 397804287
Test: m
Change-Id: I14defec8200c9c1b567b0218030277d14d41664b