Allow network scorers to provide an opt-in activity.

A candidate scorer may have an activity registered to listen for the
ACTION_CUSTOM_ENABLE intent action. If so, when the user attempts to
select this scorer as the active one, this custom activity will be
launched and will be responsible for requesting that the framework
activate the scorer. This is to enable more complex opt-in flows prior
to becoming the active scorer.

Scorers which do not specify this activity will default to using the
framework switcher (currently ActiveNetworkScorerDialog, though this
may change).

Note that all switches must still go through the framework switcher;
the opt-in activity simply allows scorers to first show another screen
and perform the framework switch on their own terms.

Bug: 15775314
Change-Id: I3847e5d3161a8fcc9622abc03218795697c32778