Only show the card if we haven't already enabled an assistant, and if
there's one available on the device to enable. (For now, we enable the
first one; in the future, we can offer a selection between
assistants).
When the user opts in, we either show the standard system activity or
the custom opt-in activity if the assistant has defined one. In either
case, we only dismiss the card if the child activity indicates that
opt-in was successful.
Bug: 16376277
Change-Id: Ic92c7676c563ab8c8fb695117a053004f15f7d25
873fa4f2ba