Add new DialogFragment and Controller for capability discovery opt-in

Adds a new controller to monitor the capability discovery opt-in
setting as well as a new DialogFragment, which displays a dialog
providing the user with more information before they enable the
setting.

Also removes multiple updateSubscriptions() happening when the
activity is first created from onStart() and onChanged() callbacks.

Bug: 111305845
Test: manual
Change-Id: I70821964bc618c3c389c9039cd7f5028e34c7ebb