update_engine: removes unneeded dependencies from unittests

Removes dependecny to network_proxy and libcros in unittests as they are
already dependent on libupdate_engine.

Fixes some lint errors of update_engine.gyp.

BUG=none
TEST=cros_workon_make --board=amd64-generic --test update_engine

Change-Id: I18bb9c77b49cab70075a622700549da602c52e5d
Reviewed-on: https://chromium-review.googlesource.com/636104


Commit-Ready: Amin Hassani <ahassani@chromium.org>
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Ben Chan <benchan@chromium.org>