Add a helper script to create an AGL build repo

AGL server build requires checking out a few Android and open source
components via repo. Doing this from a local checkout of Android is
possible, but not trivial.

Add a script to automate those steps.

Bug: 155491613
Test: ./newrepo.sh ~/test_repo
Change-Id: I602fd2048bc979836c494abb64d6f5c24326bc99