[modular][config] Manually parse strings to enums

XDR expects an integer for enums so this updates modular_config_xdr.cc to
manually parse the config file for any fields that need to be converted to
enums.

This also moves story_shell_url to basemgr because basemgr is currently passing
it through to sessionmgr.

Change-Id: I19bf734132b1b8f6fada93f3b754a010554ca29b