Make ProblemNotificationDialog public again

There shouldn't be any need for a client in accessing the dialog
directly but at least SayMore and Solid do, so I'm changing this
back.

A better way for the clients would be to call
ErrorReport.NotifyUserOfProblem() instead of instantiating this
dialog.

Change-Id: I86aedd9d306a45fffffad729260810f8c2585f51