update_engine: Update unit tests to use unique_ptr for PolicyProvider

The class policy::PolicyProvider from libbrillo has been updated
to accept unique_ptr instead of raw pointer for constructor.

This CL updates the tests to use the new constructor.

The changes in PolicyProvider contructor come from CL
https://chromium-review.googlesource.com/c/aosp/platform/external/libbrillo/+/674935

BUG=chromium:764337
TEST=cros_workon_make --board=${BOARD} chromeos-base/update_engine --test
CQ-DEPEND=CL:674935, CL:819250, CL:819353

Change-Id: I5537cfe9a88053193fbe8b0b6d844c59df148fc9
Reviewed-on: https://chromium-review.googlesource.com/819412


Commit-Ready: Igor <igorcov@chromium.org>
Tested-by: Igor <igorcov@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Sen Jiang <senj@chromium.org>