biod_proxy: DBus proxy library for Biometrics Daemon

Note that only boards with the biod USE flag build the biod package. However, other programs, e.g. cryptohome, may need to link against a dbus proxy for biod on all boards. Since the dbus proxy for working with biod needs to handle an “AuthSession”, a dbus proxy generated with platform.eclass's platform_install_dbus_client_lib function is not enough. The biod_proxy package (with its own ebuild) provides a library, so that all boards can include the proper proxy for working with biod, but only boards with biod USE flag will actually build and run biod.