audio: new routing strategies and stream types

Added new routing strategies and stream type for internal use
by audio policy manager and audio flinger:
- One for accessibility to allow different routing than media
- One for re-routing (remote submix) in preparation of dynamic
policies
- Added stream type for "internal" audio flinger tracks used
for audio patches and duplication.

Bug: 18067208.
Change-Id: I88f884b552e51e4a49c29125e5a1204cf58ff434