Bug 969672 - Handle declined engines in meta/global.
Review comment: accept null in setDeclined.
Review comment: explicit naming of declineEngine/declineEngines.
Review comment: make getEngineNamesFromPref protected.
Review comment: make, not get.
Review comment: test entire engine name set.
Review comment: fix comment.
Review comment: use set equality instead of testing for membership.
Review comment: don't allow SyncConfiguration.declinedEngineNames to be null.
Bug 969672 - Part 2: add propagation of user-set declined engines.
Rework declinedEngineNames.
Bug 969672 - Part 3: merge local and remote declined engines when processing a downloaded meta/global.
Bug 969672 - Part 4: add or remove engines from the declined list when we detect that they've changed.
Add declined engines to newly generated MetaGlobals.