when one is not supplied in modular config.

- AutoLoginBaseShell will either login as an ephemeral or persistent
"guest" user when launched by basemgr.

- DevBaseShell is explicitly meant to be used in tests, even if the
functionality is similar.

USAGE: specify base_shell=fuchsia-pkg://fuchsia.com/auto_login_base_shell#meta/auto_login_base_shell.cmx and optionally specify base_shell_args=--persist_user

TEST: fx run-test auto_login_base_shell. Test will validate that using auto_login_base_shell
automatically logs in the user to session shell

MF-343 #done

Change-Id: Ice2406ff467cf0ef366236b6c04b831f5fd8a1fb