When the handoff-to-awesomebar feature is enabled, about:home doesn't need to get the data about all search engines, only the default one.
ContentSearch/ContentSearchChild doesn't have an API for that at moment, only getState which gets information about all engines, serializes it to send to the child, only to be unused. So we need to add a new getCurrentEngine API (or add a paramenter to getState), and make Activity Stream inform the ContentSearchUIController to see if the handoff is enabled.
(B) Even with the handoff disabled, it would be nice if initially it only got info about the current engine, and later it requested the info about all engines whenever it needs to build the search one-off buttons UI.
Spending time on (B) would depend on how likely the handoff is to become the official UI or if the previous UI will remain around for longer