libaudiopreprocessing: Cleanup PreProcessing.cpp

- Removed unused elements from preproc_session_s
- Fixed initialization of frameCount in PreProcessingTest.cpp
- Print number of frames processed in PreProcessingTest.cpp

Bug: 182583445
Test: atest EffectPreprocessingTest
Test: preprocessing/tests/build_and_run_all_unit_tests.sh

Change-Id: I4a8fb5796451eb2b3a37d6e4bd48452bccac9318