Bug 969672 - Part 1: handle declined engines in meta/global.
Bug 969672 - Part 2: add propagation of user-set declined engines.
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.
Bug 969672 - Part 5: add declined engines to newly generated MetaGlobals, overwrite declined for those updated in place.
Bug 969672 - Part 6: when selecting data types, have history map to forms, too.
Bug 969672 - Handle declined engines on Android. r=nalexander