Bug 993084 - Delay initialization of telemetry experiments if there is no active experiment. r=bsmedberg

To sync experiment state with the addon manager, we need to start it early when there is a
active experiment.
However, initializing the telemetry experiments system too early can lead to performance
regressions, so we delay the initialization if we don't have an active experiment.
Bug 993084 - Consolidate pref name constants for experiment tests. r=bsmedberg
Bug 993084 - Fix experiments health report provider test. r=bsmedberg