Add AppFunctionService.

- Ported from implementation in AppSearch.
- Simplified not using wrapper on top of ExecutionAppResponse.

Bug: 359983784
Flag: android.app.appfunctions.flags.enable_app_function_manager
API-Coverage-Bug: 357551503
Test: none (deferred with adding the CTS for executeAppFunction)
Change-Id: Ic8b53585269be1be7738a654511dcb25211074fb