Add preloaded fallback for App Functions agent allowlist.

Introduces a fallback mechanism for loading the App Functions agent allowlist. The allowlist is now determined with the following order of precedence:

1.  The value from the `allowlisted_app_functions_agents` DeviceConfig flag.
2.  If the DeviceConfig value is invalid, the last known valid allowlist from storage is used.
3.  If DeviceConfig is not set, or is invalid and there is no stored value, a new preloaded static allowlist from the `config_defaultAppFunctionAgentAllowlist` resource is used as a fallback.

Tests are updated to reflect this new loading logic.

Test: atest FrameworksAppFunctionsTests
Flag: android.permission.flags.app_function_access_service_enabled
Flag: android.permission.flags.app_function_access_api_enabled
Bug: 439532525
Change-Id: I364844d68dc5d4e7dbadac0e1f48b3cdc126cedd