audio: Change LOCAL_MODULE_PATH to LOCAL_MODULE_RELATIVE_PATH

To comply with multi-arch build system changes for 64-bit support,
LOCAL_MODULE_RELATIVE_PATH needs to be used.

Drop explicit use of LOCAL_MODULE_PATH, and use relative path to
avoid build system to overwrite one of multi-arch library variants
with the other arch variant.

Change-Id: I21f9b952a79c3930b15ff5549ac3c21ca827cc3a