Fix audio policy configuration for generic targets

Changes in this CL:
1. Use ":=", instead of "+=", for PRODUCT_COPY_FILES so previous audio
policy related settings won't be overwritten.
2. Use basic audio policy XML files provided by audio team as default.

Bug: 62524944
Test: check if the XML files for audio policy configuration are under
      /system/etc and devices with generic system imagei, but without
      such configuration under /vendor/etc can boot with audio working.

Change-Id: I4ccffdfb131a9d2445d9c2f52a22ffcc97884479