update_engine: add omaha_request_action_fuzzer target

It fuzzes the XML response sent from Omaha.

It also refactored .gyp file to separate fake*/mock* libraries from
unittest* themselves.

BUG=chromium:906815
TEST=cros_fuzz

Change-Id: Ic7d0e7d18784e48f4e43b538f9797e5d2d452d08
Reviewed-on: https://chromium-review.googlesource.com/1344914


Commit-Ready: Xiaochu Liu <xiaochu@chromium.org>
Tested-by: Xiaochu Liu <xiaochu@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>