FL7: Add MIRROR_PAD CPU reference implementation and functional testing am: e7253cf29b

Original change: https://android-review.googlesource.com/c/platform/packages/modules/NeuralNetworks/+/1956741

Change-Id: Iccfd27ee23f011b02f81847f7b772742af741fbf
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>