Bug 975000 - Disable updating and compatibility checking for Experiments; r=Unfocused

Experiment add-ons are installed and updated via the Experiments Manager
service. With this change, the Add-ons Manager lets experiment add-ons
play by their own rules without interference.

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extra : rebase_source : 12f990198fed7171b9572c24aa2f8319620414a2
Bug 985084 - Experiment add-ons should be disabled by default; r=Unfocused

Experiment add-ons are now disabled by default on application load. It
is up to the Experiments Manager to enable them.

This means that experiments may not be able to reliably collect data or
modify behavior close to application startup. (There is a window between
when the Addon Manager initializes and when the Experiments Manager
initializes.) This window is acceptable for the initial version of the
experiments feature.

The Experiments Manager doesn't currently enable experiments on startup.
This will be addressed in a subsequent patch. Its tests do not regress
(indicating a lack of test coverage), so no harm no foul.

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extra : rebase_source : 00a55146576f490200a6148bd5516f84def9879c
extra : source : bd901384b973c7ff581fe24a3012f0191b8bbe67