Activate submodules

This change moves further towards ensuring Git can understand repo's
submodules. 'submodule init' is used to make the submodules active[1].

[1] https://git-scm.com/docs/gitsubmodules#_active_submodules

Change-Id: I0c20ff1991101fc5be171e566d8fb644aab47200
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/446182


Tested-by: Kaushik Lingarkar <kaushikl@qti.qualcomm.com>
Reviewed-by: Nasser Grainawi <nasser.grainawi@oss.qualcomm.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Mike Frysinger <vapier@google.com>