audiopolicy: Convert some of .mk files to .bp files

- Renamed libaudiopolicyengineconfig to libaudiopolicyengine_config
  in order to match names libaudiopolicyengine_{common|interface}.

- Made libaudiopolicyengine_config static to avoid .so overhead.

- Added a cleanspec for it.

- Added libaudiopolicyengine_common which includes source files
  previously included directly into engineconfigurable and
  enginedefault libraries.

Bug: 121208203
Test: make & flash crosshatch
Change-Id: If3b7b89fcbd5752ddfdeca6c164a88f55bc35811