Make CpuActivation stateless

- Rename NEActivationLayer to CpuActivation
- Add member function to generate execution window

Partially Resolves: COMPMID-3992

Signed-off-by: Georgios Pinitas <georgios.pinitas@arm.com>
Change-Id: I4e1ae15cf456b860d3080b2fedc4dbcce7d1bb79
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/4791
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michalis Spyrou <michalis.spyrou@arm.com>
diff --git a/src/runtime/cpu/ICpuOperator.h b/src/runtime/cpu/ICpuOperator.h
index bb7de1b..70ab436 100644
--- a/src/runtime/cpu/ICpuOperator.h
+++ b/src/runtime/cpu/ICpuOperator.h
@@ -30,7 +30,7 @@
 {
 namespace cpu
 {
-using ICpuOperator = arm_compute::experimental::INEOperator;
+using ICpuOperator = experimental::INEOperator;
 } // namespace cpu
 } // namespace arm_compute
 #endif /* ARM_COMPUTE_ICPUOPERATOR_H */