Add --android argument to the init-repository.py script

This option initializes the upstream chromium repository
and additionally to the submodules needed for the matching
OS it also clones submodules needed for Android.

Change-Id: I1df84973aabc10f5c839bc4e3db7c6024f2c5145
Reviewed-by: Zeno Albisser <zeno.albisser@digia.com>