Add FusedActivationFunc enum and renamed certain operations.

  - Add "_2D" to CONV, DEPTHWISE_CONV, AVG_POOL, MAX_POOL, and L2_POOL,
    since they are only supposed to do 2D computation.
  - Remove CAST, GATHER, SPLIT.
  - Also update the tests to reflect the changes.

Bug: 63905942
Test: mm
Test: NeuralNetworksTests pass for both CPU path and sample hidl driver.
Change-Id: I5bcf219c4c6cd4f866c1023d1f35cdc83ca69a85