Update mojo shell so that --args-for can be used on android

This refactor the shell startup to factorize the parsing of --args-for argument
so that it can be reused on android.

It also changes the way android gets its application argument. Instead of
expecting it as a specific intent parameter, it does exactly as the desktop
version and looks for it in the generic argument list.

R=sky@chromium.org

Review URL: https://codereview.chromium.org/816473002