Rework PulseAudio / audiomanager config setup to allow over-rides

Changes include:
- Move from having PulseAudio configuration changes in the recipe
  bbappend as patches to using the .include directive in default.pa
  to pick up configuration fragments provided by other recipes.
- Use virtual/pulseaudio-config to pick up the previously hard-coded
  configuration for enabling module-router.so, which is now provided
  by a new recipe that sets RPROVIDES appropriately.  This allows
  over-riding the desired configuration if needed.
- Switch the audiomanager-plugins-config dependency to using
  virtual/audiomanager-plugins-config so that it can be over-ridden
  with PREFERRED_PROVIDER if needed.

Bug-AGL: SPEC-1025, SPEC-1026, SPEC-1027

Change-Id: Ifdf16871cddc2bec7761b8c63239623b63c9e984
Signed-off-by: Scott Murray <scott.murray@konsulko.com>