Bug 989137 - Part 8: Prevent unknown experiments from being installed; r=Unfocused

The experiments service insists on being in control of experiments.
Before, it wasn't being as assertive as it needed to be: other browser
components could install experiments behind its back. This patch
reasserts the experiments service as king of experiments add-on
management.

--HG--
extra : rebase_source : 91439622e47419fae7675d83890e83cf37db453b
Bug 989137 - Part 9: Ensure add-ons are enabled when they should be; r=Unfocused, r=gfritzsche

Experiment add-ons are disabled by default in the Addon Manager. This
patch will manually enable them if necessary.

--HG--
extra : rebase_source : 2271372b232b5d85af32a4a96f2e3beb867cabf9
Bug 989137 - Part 10: Add a testing-only JSM for common AddonManager operations; r=Unfocused, r=gfritzsche

There is a lot of boilerplate testing code that performs common
AddonManager operations. Some common operations used for testing
Experiments have been refactored into a testing-only JSM that lives as
part of the Add-ons Manager.

--HG--
extra : rebase_source : f18cf54c7af6d390ed4437bd0ed0eb4beea76b0d