Relax NeuralNetwork's VTS positive and negative base tests

There are some NN VTS tests that assume a service is able to generate a
model consisting only of a floating point add operation. However, some
drivers do not support floating point operations. This CL relaxes the
test requirements to allow a test to be skipped if the service does not
support floating point add.

Bug: 77227504
Test: mma
Test: VtsHalNeuralnetworksV1_0TargetTest
Test: https://drive.google.com/open?id=1P-rEzUMK0EoWSrOFuXy46EubpwC3A1cs
Change-Id: Ied49c96af975eff734377b7b7445c6f5e63e117e
(cherry picked from commit 25d9dd7346d995b40e1d370eb9d12bb7e3f03396)