NNAPI narrow evaluation for P -- HAL

We have determined that for Android P it is sufficient to have a
mechanism for a developer to specify on a per-model basis that it is
acceptable for FLOAT32 operations to be carried out as if they were
FLOAT16 operations. This CL extends the model to include information on
these precision requirements.

Bug: 63911257
Test: mm
Test: NeuralNetworksTest
Test: VtsHalNeuralnetworksV1_0TargetTest
Change-Id: I5331e69c05c20c588035897322a6c7dceca374a0