Code changes for audio offload support

Code changes for audio offload support

- Share existing offload output if compatible instead of
  creating a new one. Hardware has limited number of
  offload channels (possible only 1) so we must use as
  few as possible

- Changes to routing handling for offloaded outputs

Change-Id: I11111d8f504828595bd8dbf66444cbd1202e7192
Signed-off-by: Eric Laurent <elaurent@google.com>