Make NNAPI countNumberOfConsumers return GeneralResult -- hal

Previously, countNumberOfConsumers would trigger a CHECK if the input
was invalid. This CL makes countNumberOfConsumers gracefully fail on
errors, instead returning the error through the GeneralResult.

Bug: N/A
Test: mma
Change-Id: Iee54f87768e52fdf701c22d94083c053b881733d