Avoid constraint suicide during ObjectDestroyed callbacks

Avoid constraint suicide during ObjectDestroyed callbacks

Valgrind reports this when the gallery grid layout terminates:

==5351== Invalid write of size 4
==5351==    at 0x4F8FF32: Dali::Internal::ConstraintBase::ObjectDestroyed(Dali::Internal::Object&) (constraint-base.cpp:218)
==5351==    by 0x4F962B9: Dali::Internal::Object::~Object() (object-impl.cpp:1206)
==5351==    by 0x4FA810B: Dali::Internal::CustomObject::~CustomObject() (custom-object-internal.cpp:110)

The issue is that constraints commit suicide during ObjectDestroyed callbacks,
with a (dangerous) call to mTargetObject->RemoveConstraint.

Change-Id: I8da276fda1b4dfd987a8bd699ed3d908994b361e