Extract AppFunctionServiceCallback from AppFunctionManagerServiceImpl

AppFunctionManagerServiceImpl is quite a large class. Making it modular for better readability.

Change-Id: Ibaef088e55c7121d65df85087a7e115873f9dfe6
Flag: android.app.appfunctions.flags.enable_app_function_manager
Test: atest CtsAppFunctionTestCases -c
Bug: 360864791