Replacing LOCAL_COPY_HEADERS with LOCAL_EXPORT_C_INCLUDE_DIRS, for VNDK

Moved msm8974/mm-core header files from inc/ to inc/mm-core/omxcore for exporting.
Moved msm8974/mm-video-v4l2/DivxDrmDecrypt headers to subfolder for exporting.
Moved msm8974/libstagefrighthw headers to include/ folder for exporting

In msm8974/mm-core, inc/ has newer OMX_* headers than inc/mm-core/omxcore and
frameworks/native/include/media/openmax. Updating these headers will be
handled in a separate CL.

Test: Add lib to LOCAL_EXPORT_SHARED_LIBRARIES
Change-Id: I4cdfe3085a459abdbd3de491eed10cdceadc4a85