Change the semantics of RefTensorHandle::Import to 'overlay' existing memory

This makes it possible to call Import on an Allocated() or memory-managed Tensor,
which is needed for the current implementation of OptimizerOptions::m_ExportEnabled
to work (as the last layer before the OutputLayer needs to be able to Import the
user's OutputTensor, but this is done after other memory allocation).

Signed-off-by: Matthew Bentham <matthew.bentham@arm.com>
Change-Id: I1a885c2da7b1f0f3964ae53b8135b5e96a66614f