Mojo: JSAppShell ConnectToApplication service_provider parameter can be a ServiceProviderProxy

Make shell.connectToApplication behave the same way that generated JS methods do wrt interface parameters.

The value of the connectToApplication service_provider parameter can be a Mojo handle or a ServiceProviderProxy (the latter is much nicer of course). The hello.js example has been updated.
BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/760593004