Introduce condition provider services.

Add the condition provider interface, base class, and associated
system metadata.

Pull out common service management code into a reusable helper,
used by notification listeners and condition providers. The
helper, ManagedServices, is now completely self-contained - it
has no dependencies on NoMan or NoMan abstractions.

Bug:13743109
Change-Id: I6856d40f0a2ead78ac9b5707568559a57e7eb009