Cleanup NNAPI VTS tests

This CL includes the following cleanups:
* namespace compression
* remove "using" from header files
* remove no-op code, default no-op constructors
* clang-formats the code

Bug: N/A
Test: mma
Test: VtsHalNeuralnetworksV1_*TargetTest
Change-Id: I023997d8686ca65223858eed3a0881f5444ed2d6