Make NNAPI CTS tests better utilize multi-threaded compilation.

The source file (tests/tests/neuralnetworks/src/TestGenerated.cpp) was
including frameworks/ml/nn/runtime/test/for-cts/TestGeneratedOneFile.cpp
which includes all generated test cases (.cpp) files directly.

In order to better utilize multi-threaded compilation, this change
directly adds all generated tests. And also removed the unnecessary
trampoline source files.

The reason why the compilation is done in two steps is to make sure the
build system able to pickup AndroidTest.xml config file correctly.

Bug: 118337512
Test: mm
Test: CtsNNAPITestCases
Change-Id: I413029208de9b30905ab272a79fff6a31ccb4da6