IVGCVSW-2102: Fix Pooling2D CpuRef indexing bug

IVGCVSW-2102: Fix Pooling2D CpuRef indexing bug

* Fixes bug when calcuating indexes for NHWC in
  Pooling2D CpuRef implementation, it now uses
  TensorBufferArrayView.
* Adds 2-Channel unit tests for Pooling2d on CpuRef,
  Cl and Neon. The single channel tests were not
  properly exercising Pooling2d using NHWC data layout.
* Refactors Pooling2D NHWC tests so that the input and
  output data are permuted to NHWC when necessary,
  instead of hard coding the data in NHWC format.

Change-Id: I5b9d41ed425ff283ea8c8ef6b1266ae0bc80f43b