update_engine: Rename policy_manager.conf file to an example configuration

The actual configuration is now installed by the virtual update-policy package.

BUG=chromium:352739
TEST=build update_engine

CQ-DEPEND=CL:199413

Change-Id: I6db9b2b6603de6e972bc4492a31d3a3baf42837b
Reviewed-on: https://chromium-review.googlesource.com/199421
Reviewed-by: Alex Deymo <deymo@chromium.org>
Tested-by: Gaurav Shah <gauravsh@chromium.org>
Commit-Queue: Gaurav Shah <gauravsh@chromium.org>
diff --git a/policy_manager/policy_manager.conf b/policy_manager/policy_manager.conf.example
similarity index 100%
rename from policy_manager/policy_manager.conf
rename to policy_manager/policy_manager.conf.example