Bug 973992 - Telemetry experiments: Make the experiments code use the new experiments addon type. r=Unfocused

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extra : rebase_source : 54de4767390441a895fca148c2c79483f22c1366
Bug 986040 - Telemetry experiments: Assure that no experiment addon is running yet before starting an experiment. r=Unfocused

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extra : rebase_source : 55be48f0b9cffe978c22baa8260b161f2cbc0562