Add conditional class starting

This adds the ability to prevent a class from starting
if a certain persistent property has been set to
disallow it.

A class will only load if there is not a property named
persist.init.dont_start_class.[class name] set to 1.

Test: Set a property called persist.dont_start_class.[class]
      to 1. Verify that the given class does not start
Change-Id: I51c70ad635762ed77855d0509e630adb0aec0eb1