COMPMID-3443: Android R CTS 1.3 QASYMM8_SIGNED failure with MEAN on CpuAcc

COMPMID-3443: Android R CTS 1.3 QASYMM8_SIGNED failure with MEAN on CpuAcc

- Properly perform the division of the sum of the elements by the interested dimension of the tensor

Change-Id: I0a30be4e5e6cfc1bd24a17eb4307dd2acf98db3e
Signed-off-by: Manuel Bottini <manuel.bottini@arm.com>
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/3145
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Georgios Pinitas <georgios.pinitas@arm.com>