Add a new setting key APP_FUNCTION_AGENT_ALLOWLIST_ENABLED

It is enabled by default.

For local development, a developer can turn the agent allowlist check off to test a not-yet-certified agent app.

Bug: 421113140
Flag: EXEMPT just a setting key
Test: presubmit
Change-Id: Iee7c775dbb989315d2e112ea104b0a1dbd46f8d4