NNAPI: Clarify behaviour of ops that reduce dimensions

Clarify behaviour of the following ops in the case when inputs would
cause an output to have an empty shape:
* MEAN
* ARGMIN/ARGMAX
* STRIDED_SLICE

Bug: 155508675
Bug: 155660285
Bug: 155508675
Bug: 155238914
Test: VtsHalNeuralnetworksV1_3TargetTest
Change-Id: I92d1c3866a462a99b4eed7782b72f62bb6a076c6