libeffects: Rename .c files at lvm/lib to .cpp

Removed extern "C" declaration in header files. Since all
files at lvm/lib are .cpp and all function names are unique,
extern "C" is no longer required.
Note: Build errors due to renaming of files to .cpp
are taken care in this patch.

Test: android.media.cts.AudioEffectTest
Test: android.media.cts.AudioPreProcessingTest
Test: android.media.cts.BassBoostTest
Test: android.media.cts.EnvReverbTest
Test: android.media.cts.EqualizerTest
Test: android.media.cts.LoudnessEnhancerTest
Test: android.media.cts.PresetReverbTest
Test: android.media.cts.VirtualizerTest
Test: android.media.cts.VisualizerTest
Test: Play Music offload / non-offload
Test: Tested using standalone application
Bug: 129491957
Change-Id: I7033146c21681372a212d507f959ab02568b7538