Update acc type in AVG_POOL2D operator

- Update eval functions of AVG_POOL2D operator to use acc_t,
  consistently in intermediate operations, including subtracting the
  zero point from input  in int8.
- add unit tests to catch regressions in accumulator behaviour in
  AVG_POOL2D.

Signed-off-by: Eirene Vlassi Pandi <eirene.vlassipandi@arm.com>
Change-Id: I0f8796c8a066918aae7d699fa4cd8c52dd88811a