Remove test_com.android.neuralnetworks from apex_available

In order to simplify the apex logic in Soong, avoid needing to mention
test apexes in apex_available by setting apex_available_name in the
test apex to masquerade as the corresponding non-test apex.

Bug: 383592644
Test: builds
Change-Id: I1f222727c37200ae661e9671360c57cbd38f0009