testing/anbox: upgrade to 20200408, drop lxc dependency and fix init script

drop lxc dependency, as apk detects the lxc-libs dep and the init script creates the required directory itself and the dependency is not needed otherwise.

fix the init script: when the container manager failed, the openRC service wasn't able to start. It could not be started again because anbox-bridge.sh errored out: it was unable to create an already existing interface.