Renaming NEActivationLayerKernel to CpuActivationKernel
Renaming NEActivationLayerKernel to CpuActivationKernel

Signed-off-by: Nikhil Raj <70dde441576cd8b053dd4c3a595f3f0b9c7a63a3@arm.com>
Change-Id: I33dc1127627b4b89c02a3bea246eaf7499e5a780

ARM-software/armnn,ARM-software/armnn,ARM-software/armnn