Implemented the Handle assignment operators properly

[Problem] Some unsafe handle assignments are allowed by the compiler e.g.
Actor notAnImageActor = Actor::New();
ImageActor notSafeToUse = notAnImageActor;
[Cause] using BaseHandle::operator= is unsafe
[Solution] Implemented the Handle assignment operators properly

Change-Id: If4cd798d2cb394ac4dbac96b1c17598a2aae8ee6