Persist AppFunctions agent allowlist to disk.

This change introduces a file-based storage for the AppFunctions agent allowlist. The allowlist fetched from `DeviceConfig` is now persisted to `/data/system/appfunctions/agent_allowlist.txt`.
If parsing the allowlist from `DeviceConfig` fails, the system now falls back to the last known good version from this file, making the feature more resilient to invalid configuration.

Flag: android.permission.flags.app_function_access_service_enabled
Test: atest FrameworksAppFunctionsTests
Bug: 416661798
Change-Id: I64969dbda90501a132dab5aa11186a1305a7ebe6