Add FLAG_ACTIVITY_NEW_TASK when prompting about carrier app.

This should never have worked, but appears to on P due to some
compatibility hacks. This context is not an activity context, so any
activities it starts must be in a new task.

Bug: 117248944
Test: See bug comment #9
Change-Id: I9cd8841327f76b3675f2fd3b880e4d71c9089e1c