Add flag extend_vb_chain_to_updated_apk as read-only

Due to a limitation in the current flag infra (b/299647580; device
config not yet initialized for the code to be flagged in package
manager), the only way forward is to make the flag read-only. It
effectively turns the flag into a build-time flag, and remains to be off
until Gantry ramp-up.

Bug: 277344944
Test: Use the flag in package manager, it doesn't crash in the flag infra
Change-Id: I059980ca0e15c24b54c3fece7bc69a8301246c7a