Android backend: fix multi-output rules.

Rules that generate multiple output files (like protoc) need the same
workaround as actions; the dependency between the output files must have
an empty command to force make to believe that the mtime may change.
Copy the workaround from WriteActions.

BUG=
R=benm@chromium.org

Review URL: https://codereview.chromium.org/15300013

git-svn-id: http://gyp.googlecode.com/svn/trunk@1632 78cadc50-ecff-11dd-a971-7dbc132099af